I agree that the capability to flash alternate OS would be nice, but manufacturers supporting it would be just a wishful thinking as we saw in the smartphone ecosystem; manufactures which supported aftermarket firmwares when their were new to market (to seek hacker/developer blessings) eventually went on to lock down their phones once they reached top.
But open-source Smart TV OS is gaining prominence in the form of webOS OSE - https://www.webosose.org/ ; I hope there will be soon a new TV entrants with webOS OSE based firmware.