Don't think the 3 will see a steep devaluation because of the Y. There will be a market for both. The S and X look different and co-exist. The Y isn't all that big and I always worried in 2 years people who need the storage and seating for more than 5 they would get more use from a used X which will be able to be had under 50-60k easily. Variety of choices will be good for the consumers. Some love the 3 design, others like the Y. I personally own a X for the storage but prefer the 3 because of the smaller size and fun driving dynamics and I think the whale rear design of the X and Y just don't look aesthetically pleasing to me. For a stop gap vehicle, I can't realistically believe the SR paired with tax credit will lose all that much money and is a great car. However my price predictions along with most 2018 buyers turned out to not be a wise or smart move so anything can happen... Wish I had waited a year to buy my 3 and X as I just got it early just to make use of the tax credit but it turned out to be a huge 25k+ mistake in price drops!
If you look at general sales figures of sedan versus hatchbacks, it's clear what most people prefer.
Model 3 is a success not because it's a clumsy hatchback, but in spite of it, due to the tech underpinnings and the Model S and X having so much (high cost, novel) image.
I bet that many of the early adopters, those who stood in line or bought one of the first 200,000 Model 3, will want to upgrade to the Y. They got a more pricey Model 3 to begin with after all, they can swing it, no-problem.
With Model Y hitting the market at only a small premium over the 3, I expect new sales to immediate below 1:2, even 1:4 unless they do something special to keep Model 3 relevant to some audience(s). Introduction of a Ludicrous version with the upcoming bigger PM motor to go into the S and X, for instance. If they want to, they could probably make Model 3 pull as hard as SP100DL does today. Bit lighter, bit smaller battery, it evens out if they can (choose to) amp it up for just 3 seconds.
Model 3 losing popularity in new sales can't help price retention of a huge supply of used ones. And if the Model SR can be had relatively short term (with good V3 charging), why would you pay a s similar amount for the 2017-2018 Dual Motor (Ps) that only offer range, not utility?
I suppose I hope for early adopters it won't be too painful a depreciation for them, but then, that's what being an early adopter is all about. Bragging rights don't come cheap.
In my opinion the Y is vastly bigger in everyday reality. Isn't it 66 vs 25 ft³ cargo space? The Model 3 is just exceptionally awkwardly packaged, even for a sedan. The BMW 3-series is aimed to defeat foremost, has significantly better cargo space despite the lack of a frunk.
Also, in the Y you get higher seats which should vastly improve seating comfort especially in the back. Tesla's flat floor system is nice and cost effective but other brands are already seeing ways to put batteries under the second row or in place of the "drive shaft tunnel" to get a comfortable (large) seat to foot bed height. Sitting in the back of an X is vastly better than in a 3, due to the seat height. People care for rear seat comfort, if they're not early adopters.
Looking a few years into the future, Model 3 could become more of an entry level car as economies of scale allow. While using the low stance to offer motorsport inspired cars where the rear seat is just not a factor at all. Ludicrous. Wing game. And the car would actually be a great platform for it, seeing track times fom the stock Performance car, let alone with (frankly, needed), mods.
A Performance Y then, is a bit daft if you ask me.
