Sure, that was my point. Once the 18650 format no longer provides the best energy density and lowest cost then Tesla may lose their battery advantage. Obviously I don't expect this to happen any time soon but it is a real possibility at some point in the future. Remember Tesla ends up going from a cell density of around 250wh/kg to a pack density of 150wh/kg because of all the support the 18650 Cobalt based cells require. A cell chemistry that requires less support could come in at a lower cell level wh/kg but end up at a higher pack level wh/kg. A non O2 containing cell with a non flammable electrolyte for example might have no concern at all for thermal runaway and might have no need for active thermal management.
I agree that might be a factor in the future, but would like to put it in perspective:
a) although the ideal battery chemistry might perhaps not require as much thermal management, we are a far way from approaching the ideal battery chemistry. This means that any improved battery chemistry, be it the next big thing, or the big thing after that, may again require lots of thermal management. It would be a significant design restriction in developing new battery technology, if thermal management wasn't available. Tesla will likely continue to have more choices. Even if it is possible that during some period of time, it might be as you say, that period might not last very long. And it doesn't seem to be on the horizon, in any case.
b) as others have said, we can expect Tesla to continue being innovative, and not be a company that innovates once and then only wants to improve their past innovation. New battery technology might have other problems (or even additional ones), and Tesla might again be the company finding the best solution first.
In other words, I agree that there is that risk, however it is not something that will inevitably happen sooner or later. And of course, there are many other things that make Tesla a great car company. The big car companies may be able to throw a lot of money at something once they get in motion, but looking at it only from that point of view wouldn't do justice to Tesla.