I think the Tillman addition is also a potential long term plan by Brad Stevens, more so the "Horford replacement". Both guys are similar builds. Tillman is 2 inches shorter but he's obviously younger and has a higher wingspan.
I like the idea of Tillman, Queta and Kornet being our frontcourt depth beyond 2025 with Porzingis of course as the lead dog in that group. Yes, Queta is not likely to be a major contributor for our postseason run this season, but he's still raw/developing. Maybe it's hopeful projection on my part, but I think with some more experience and 1-2 full offseasons, Queta can develop into a very solid role player/fringe starter. He has the tools, size and toughness to make it in this league.
Look at Hauser. Two years ago many just thought he was another "bum" but after playing a handful of games, and a few full offseasons of work/training, he's turned into a very useful role player for us.
I'm admittedly not a huge Kornet guy but if he's your 4th big, you're more than fine. I think Horford has two years maximum left after this season concludes, so the hope is probably the "next wave" (Tillman + Queta) can grow into solid players in the frontcourt, and they will be cheap for the meantime which is good for luxury tax purposes. And of course Porzingis is our main big for years to come.