I agree that we will necessarily have energy storage powering the grid, the same grid trucks are drawing from. However, as a counter argument I would say it is not full duplicity because you will need far more batteries if they are installed on each truck (reduced efficiency of transport due to battery weight, reduced utilization of batteries since they are isolated from the grid when on the trucks). Also, it may be optimistic to think truck batteries will be stabilizing the grid when not installed. A truck doesn't want to pull in to a swap station to receive a 65% full battery. I would suggest transport is a very predictable and stable load. Helping reduce the battery requirements, transport consumption will be higher during business (daylight) hours, which promotes tying into the grid also.
Imagine a skateboard frame semi that has a small cab plus a rack for 1 to 6 200kWh battery packs that are easily loaded and unloaded like intermodal shipping containers. I truck pull into a swap station and loads as many fully charged packs as needed for the next leg of its trip. The swapping is all automated and takes less time than filling a tank of diesel.
So there is a large inventory of battery packs and a network of swap stations. Each battery pack is used in transit a few hours per day, the rest of the time it sits at a swap station charging and balancing the grid. So at any point in time 10% to 20% of this inventory is in transit. Another 5% is fully charged and ready to be deployed for transit. The remaining 75% or so is charging and discharging to balance the grid and to maintain the 5% inventory fully charged and available for swap.
While it may be true that the electrical load of truck could be predictable and stable, that load curve through the day is not likely to match solar or wind power generation. Particularly with autonomous ESemis it makes sense to ship things mostly at night when the highways are least utilized. So matching trucking load to solar entails use of batteries.
The way to think here is to fast forward to a time when there are no fossil fuels used to generate electricity. There is no baseload power. It's almost all intermittent power, say 60% solar, 30% wind, 10% dispatachable renewables like hydro. Globally average power consumption is about 5 TW now. In this fossil free world, you need say 10 TW of power (includes transportation) and 50 to 100 hours of storage, i.e, 500 to 1000 TWh of batteries. The fleet of semi power packs would be on order of 10% of power consumption. So if trucking had enough power packs to handle its power demand, it would need say 20 hours of storage. There is no need to have that much storage sit permanently on tractors. So the average trucking power pack spends most of its time just sitting at fixed location. The question then becomes how to make that battery pack most economically productive while it is parked. Buying power when it is cheapest is obvious. If there is an oversupply of solar power at midday, then batteries can charge at nearly zero wholesale cost per kWh. They can also discharge surplus stored energy when the wholesale price is at a peak. So this inventory also serves as a peaker plant. Thus, the energy arbitrage comes close to paying for all the energy used by the trucking industry, nearly zero net OpEx. Left is for trucking to pay is the CapEx on its battery and swap station fleet. So once you have an end state like this in mind, you can work backwards to a business model that is profitable in the present. This is what Musk does best as an entrepreneur. The shorter the path to the end state, the less capital is wasted along the way.
So the challenge for Siemens is to envision this sort of end state for the grid where there is 50 to 100 hours of battery storage smoothing out 90% intermittent solar and wind. They need to have a compelling reason to add highway electrification AFTER batteries are ubiquitous. I do not believe it is. Rather they are using the present grid and fossil generation as their starting point and asking how to electrify trucking without massive batteries. It is fine for them to bridge forward, but the risk is developing infrastructure that will outlive its usefulness. It certainly may be the best thing in some regions where hydro and geothermal can provide the vast majority of energy needed. So the thing to think about is what 100% renewable would look like in a region and how much battery storage would be needed to balance the local power market.
I'd also point out the obvious that the trucking power pack model above is not at all incompatible with Siemens system. Trucks are only loading the packs they need for the next leg. If that leg includes an electric highway, it minimizes the number of packs needed for that leg, assuming that the tractor is fitted to use the electric highway. So this means the cost of using the electric highway power must be cheaper than using battery pack power. So if it bright and sunny at the moment then the electric highway could be cheaper. Otherwise, stored surplus solar energy in battery packs could be cheaper. So trucking companies could easily switch between the two on a moment by moment basis. What this means for Siemens is that their infrastructure is only competitive whenever there is surplus renewable energy. This undermines utilization. If an infrastructure is only used about 6 hours a day will it really pay for itself? Can they convince taxpayers to foot the bill? How can they get truckers to pay a high enough rate to cover the infrastructure, when the battery option is cheaper? So a key issue here is just how much capex can be recovered from truckers over a multi-decade timeframe. The investment issues for battery packs are different. We are talking about 2000 lifecycles which create value in and out of trucking. The size of the battery fleet adjusts to demand and depletion of the battery packs. Local surplus inventory can be moved to other locations. So the full utilization of each pack is pretty much assured. But if one stretch of electric highway becomes underutilized, it is pretty much a stranded asset, and investors take a loss. The problem here is very much like the challenge that railroads have in competing with truckers. If a railway is underutilized, not much can be done. But if there are too many trucks in a given market, they truckers will just take them to another area where they can get work, and truckers are much more nimble about hauling stuff exactly where it needs to go. Especially, when the pace of change is rapid, it makes sense to bet on the most nimble technologies and flexible investments.