An important question here is "Does Starfleet even want more Galaxy Class ships?"
I don't think Starfleet would skip the opportunity of using that saucer for
something. If it's not for another
Galaxy, it could be for
Galaxy spares, or for a
Nebula, or for some other ship type that uses this type of component. However...
Oh, and yes, the Galaxy Class design is almost certainly standardized at least to the point that anybody's saucer can mate to anybody's stardrive.
I think the interesting thing would be whether the software side of the saucer would be compatible with any other stardrive. Such a complex system might have "grown to accept" only a certain specific partner, and would have to be extensively "re-educated" in order to accept a substitute.
I don't think Starfleet would be in any hurry to commission a new
Enterprise to replace the lost one. A new
Galaxy would probably be called for, though: we have every reason to think that Starfleet is satisfied with that ship type, since various flashforwards show it in continuing use, and it features big time in the battles of the Dominion War, even forming something of an elite unit in "Sacrifice of Angels" (a "
Galaxy wing" is mentioned, and appears to provide the major firepower needed to breach the Cardassian lines after the initial opening is made).
On the issue of whether the saucer would be a total loss at the end of ST:GEN, I vote yes. Whether every saucer landing would end that way, though... I don't think so. The explosion of the stardrive seemed to deprive the saucer of some key assets such as thrust and SIF protection, in addition to resulting in a somewhat premature and unexpectedly uncontrolled atmospheric entry.
On the issue of removing the crashed saucer, I agree there probably wouldn't be any hurry. Starfleet would send a mop-up team to do it with discretion: no flashy orbital bombardment, but rather a quiet dismantling. Interestingly, the fight between the E-D and the BoP, as well as the warp entry of the trio of rescue ships, appears to take place on the sunny side of the planet, making it unlikely that the people of Veridian IV would be able to see anything meaningful near Veridian III against the glare of the local sun...
Timo Saloniemi