This would be the most stupid thing that Supercell could do (along with the "gem mine" idea that was mentioned here recently).
There are already several things you can do to deal with this situation:
- Don't upgrade your heroes. Pretty simple, eh?
- Opt out of war while your heroes are being upgraded.
- Do your best without your heroes and choose attack strategies that are less dependent on them (e.g. lavaloonian or loonian) or choose less difficult opponents that you can handle.
- Gem your heroes!
- Suck it up and move on! After all, your opponents in wars likely encounter the same situation now and then so in the grand scheme of things it should all even out.

