Okay, but you're assuming that the Blues would otherwise be able to land Tavares in free agency, no? If that is the assumption, then of course trading a 1st and Thomas is an overpayment. The point of making the trade is that there IS uncertainty - 29 other teams would be bidding on Tavares.
I understand that there's a lot of uncertainty in this entire scenario - if NYI lets teams talk to Tavares, then perhaps Tavares only agrees to potential deals with a few teams, several teams, or maybe only one team. If the Blues are one of those teams, then the amount the Blues would be willing to trade depends on how many other teams have potential deals with Tavares. If the Blues are the only team that has a potential deal, the obviously the Blues have leverage, and there would be no need to trade Thomas. But if the Blues are one of a few/several teams with a potential deal, then the Blues would be bidding against other teams for a signed Tavares - in this case, the scenario becomes much more similar to your typical trade, and Thomas would certainly be on the table.