Yes they are standard gauge and can run on regular old rail and that's plenty useful, but no that's not what they were built for, it would be a complete waste of the rolling stock.
As an aside, calling an idea complete bullshit based on an nitpick is pretty weak, especially since you're wrong: when tgv started in September 1981, they only had part of the line constructed. Without compatibility, I.e. a completely dedicated system, you could've only traveled between two random towns.
It means the exact same fucking thing in every context. Dedicated means infrastructure which exists solely for high-speed train, not that high-speed trains can only exist on the dedicated infrastructure.
> It would mean complete incompatibility
That's completely nonsensical. European high-speed lines are dedicated, non-high-speed trains do not run on them, it's the high-speed rolling stock which is compatible with older electrified rail lines because they use a standard gauge and compatible/switchable electrical feed (some of the more complex rolling stock having to cross multiple borders can handle 4 different power inputs).
> This is not what EU high speed rail was built on, they use a lot of legacy lines
And they use dedicated infrastructure and have done so from the start.
> As an aside, calling an idea complete bullshit based on an nitpick is pretty weak
It's not a nitpick to point out that your statement is factually entirely incorrect.
> especially since you're wrong: when tgv started in September 1981, they only had part of the line constructed.
That was still a dedicated right-of-way which it started with, putting the lie to your statement.
> Without compatibility
That has nothing to do with your original statement. Your statement was not that high-speed rolling stock should be compatible with legacy rail and able to use it. Your statement was that european high-speed rail "definitely didn't start" with "dedicated rights of way", and further that high-speed rail only have some dedicated rights of way.