Actually, I'm about to ask if someday the testnet Bitcoin will have value in the future but I think I can answer it on my own. For some reason if someone needs testnet Bitcoin and if I have some I probably can ask for an exchange or fee. In short I will not be giving it for free. It may not expensive but at least you need to pay.
The worst part is that you are probably right, some people already tried recently to sell tBTC for mainnet
BTC on the forum....
If I can remember some Bitcoin newbies have been victims of accepting tBTC as payment mainnet BTC.
With the institutional investors going hard on Bitcoin lately, in the future there's a chance that some people will pay for tBTC. I said in the future, I once saw a thread created on this forum where the dev need tBTC and is willing to any amount requested from the owner.