It's not even "technical debt" until someone thinks "technically ... we could have done this more easily in X".
Now it's a question of developer comfort and huge investment will be made to make sure everyone building the product are having the best experience doing so.
All the old code and knowledge is now debt because it can't come with us on this wonderful journey.
It's only debt if you are paying for it going forward.
The thought should be "things would be easier going forward if we'd done X".
If it does the job and doesn't create issues then you just purchased it without taking on debt. This is easier if it is isolated/loosely coupled.
There's no point going back to improve it unless it saves you time/money in the future. If you make a better one to get the 'best experience' you are throwing away time and money for a slightly better mousetrap.
It's ironic because we're trying to build software others will depend on, meanwhile the software that we depend on tries to avoid the kind of disruptive change that accompanies rewritten-with-X... that's why we can depend on it.
Now it's a question of developer comfort and huge investment will be made to make sure everyone building the product are having the best experience doing so.
All the old code and knowledge is now debt because it can't come with us on this wonderful journey.