It was never a fully reusable design, just more reusable than before.
SpaceX plans to have no parts that are lost each flight and is working to make the tiles mostly standardized and less sensitive to faults.
No doubt SpaceX has very smart people working on this and I'm not an expert in material science, but I just find it hard to believe that same day turnaround could be possible. If true, that would really make us a confirmed space faring civilization. We could actually start colonizing Mars.
Falcon 9 has reflown in just over 4 hours [1]. (EDIT: Operational turnaround. Nozzles have been turned around allegedly without refurb in 3 weeks.)
[1] https://en.m.wikipedia.org/wiki/List_of_Falcon_9_and_Falcon_...
https://www.space.com/spacex-starlink-launch-doubleheader-ju...
They've since done two flights in about an hour with https://spaceflightnow.com/tag/starlink-9-5/ and https://spaceflightnow.com/tag/starlink-8-10/
I think the first-stage turnaround record is something like two weeks.
https://en.wikipedia.org/wiki/List_of_Falcon_9_first-stage_b...
> B1062 booster holds the record for fastest turnaround at 21 days. It launched on 8 April and again on 29 April 2022.
Seems like the actual record for turning around the same booster is 21 days, which is still quite impressive.
https://www.teslarati.com/spacex-falcon-9-new-booster-turnar...
Other factors that work in SpaceX's favor are 1) that most launches will be unmanned, meaning they can take bigger risks than the Space Shuttle program; 2) that the steel body of Starship can handle higher temperatures than the Space Shuttle's aluminum, so a compromised heat shield is more tolerable; 3) for now they have a secondary ablative heat shield below the tiles (that does have to be replaced when it gets used, but that should only happen when tiles fall off)