at some point Luhnow almost has to move some prospects because of the limitations of the 40-man and Rule 5. I'd rather not move the names you mentioned but there are going to be some tough decisions to make for roster management and the trade deadline would be a logical time to make some of those.
Which is why I think a rental ace may make a lot of sense to the Astros. You move just a couple of top prospects for a 3 month rental (instead of 3 or 4). You free up 2 spots without doing too much damage to the farm and making the end of year decisions easier. While still staying young and not committing too much salary. A contract like Hamels has the ability to be severely detrimental in a couple of years. A decent amount of risk in him switching leagues and remaining 20+ million dollar effective into his mid 30s.
You add Cueto to Kuechel, McCullers, McHugh and Velasquez. Keep the losses to just Appel and Santana.
Then next year one of Feliz, Straily, Wojo, Obie, Rodgers, etc. step up to fill some of the void left by Cueto. You don't need Appel. And the Astros have multiple copies of Santana. Or perhaps could add a bat via free agency in the offseason.
You might also want to sell a little. Feldman really has no place when he returns. His contract was front-loaded. Owed just 8 million next year. Maybe not now though, more into the offseason for Feldman. Might need to go a 6-man rotation to keep the innings down on the young guns
Edit #99:
The farm is much more able to handle a rent-an-ace than it was back in 98. I think the fears of the Big Unit fallout may way to heavily on people's minds. Plus, IMO, prospects are weighed more heavily now than back then. So you can get more with less prospects.