I think they care about the repeater tax. If they stay out this year, they are likely out of it next year and the tax will not escalate as quickly. Important as they look to sign Mobley, etc and build a team for the long term.
I think they would go into it if it's worth it but Sexton may not be worth it to them.
Besides, not sure anyone is willing to pay more than $15M - at least no one that has the space to make the offer. S&T offers rely completely on the Cavs being interested in what is coming back to them. Because Sexton is restricted and has not had an offer sheet, the Cavs don't have to make a deal for the bare minimum, etc.
I expect that they will not want to take long term salary back for Sexton unless it is a player worth their while and they probably would want at least a first rounder back with some protections.