The whole discussion around "airworthiness" of the various fleets being discussed is very subjective and depends on many factors, like how hard the jets are flown, how well maintained they are, what access there is to spares, the airworthiness management and oversight processes, etc. Basically you can have a relatively new fleet that is unairworthy versus an older fleet that is highly airworthy. Ask the Germans!
However, in the case of the T1s they are getting long in the tooth, they have been flown hard over the last 40yrs, and I suspect that supportability is now an issue. But given the OSD was pushed to 2030 only recently it makes you think someone either knows something we don't or was being highly optimistic in terms of planning assumptions....
Going forwards Hawk strikes me as a bit like the Hunter, in the best replacement for an old one is a new one! Maybe if we'd pressed buy on the original plan for 44 T2's we'd have a decent sized fleet that could be multi-tasked in the same way the T1 was over its lifespan?
Whereas now we seem to have dumped a useful asset with no clue on how (or even if) we fill the gap....