If he leaves and becomes a FA you are not left with nothing. You're left with open cap space. That means you can take raw bad money deals back with first round draft picks and only give back late second round picks that only convey if they are in the 55-60 range.
If the Cavs trade no one, they will have about $40M in open space next year if they do not re-sign their own and we only account for the first round draft pick salary.
I don't disagree with the principle of what you are saying, but that approach works only if:
1. You are below the salary cap, and
2. There are available FAs that other teams believe are worth clearing cap room to sign (and, in so doing, paying future firsts as the cost of doing business).
I'm not sure either one applies here.
Yes, the Cavs would be well below the 2020-21 cap if they let all their pending FAs play out their contracts and leave ... but there's almost no chance of that happening. I'd expect them to deal Knight and Henson at a minimum, but the only teams that would want them would be ones who are looking to send the Cavs veterans with longer contracts in return. It's also possible that Clarkson, Delly, and Zizic (also expirings) could be moved in deals that bring back longer-term contracts. Let's say the Cavs take back $40 million in salaries from trading some of those players. If TT then leaves, then they would be letting him leave for nothing, because even with him off the books, they wouldn't have any cap room to absorb some other team's bad contract.
Just as importantly, the 2020 FA class is pretty underwhelming -- the calm before the 2021 storm. I can't see a team clearing the salary decks to make a run at DeRozan or Mike Conley.
All this is to say ... as long as Tristan keeps playing well and stays away from the Kardashian girls, I'd be interested in extending him ... but if that doesn't happen by the trade deadline, then I'd be looking to sell him to the most desperate contender I could find.