The only downside that I see is that the runtime needs to be able to compile older versions of TS with all the quirks of that specific TS version, so likely it

The only downside that I see is that the runtime needs to be able to compile older versions of TS with all the quirks of that specific TS version, so likely it would need to ship multiple TS compilers. If only the ECMA Typo proposal would finally be implemented, we wouldn't have all these issues.