My personal opinion is that they never gave a promise that ALL future versions of the device will be open source and it's foolish to expect that.
While it isn't unheard of, it is fairly rare for open source projects to revert to being proprietary. Most high-profile instances of that happening have resulted in the project being forked and the original maintainers losing control of it. So no, it's not unreasonable to expect an open-source project to stay that way.
In this case, while I'm hoping that an effort to maintain the LGPL version of the Trezor firmware will take off, I suspect that the project may be too small at this point. The Trezor's install base isn't very large, and additional people likely to participate in such an effort are now unlikely to buy a Trezor.
If you don't like that buy other open source wallet
I
did buy an open-source hardware wallet. That's the basis of my complaint. ಠ_ಠ
I am happy with my purchase even if they never make another update. We can still modify any source older than yesterday's and distribute it amongst ourselves. That stupid git trick makes no difference. We have an intact GPL repo from yesterday, even if they overwrote theirs.