Fish with Marillion isn't about "bad blood", but a very simple logistical fact: Fish has been candid that he cannot sing those first four albums in the original keys, and Steve Rothery has refused (to date) to change the keys of any of those songs (though I believe they did tune down a song or two for Hogarth on this last tour, so who knows?)
The stumbling block for Kiss isn't about "bad blood" either; I have zero doubt that Ace will perform with the band at some point in this Final Tour. The problem is Peter Criss; he literally cannot do it physically at this point. This is the "Bill Ward" problem; I love Bill, and his jazz drumming is a CRUCIAL part of the early Sabbath sound (listen to the first album, or better yet, the Live From Paris material, partially released on "Past Lives"), but he COULD NOT perform what I saw in Massachusetts on that last tour five nights a week across three continents.
...my question is why would the core band members disrespect the replacement to bring back a reunion lineup - Dream Theater being the prime example.
Can't speak for DT specifically, but while there is undoubtedly cases where the incumbent is pissed off, there are just as many cases where the new guy is like "dude, if you think I'm standing in the way of Slash/Rob Halford/Tom Petersson coming back, you're sniffing too much glue." Even Steve Morse, who has been in Purple almost twice as long as Blackmore, has made it clear that if he wants back on stage with the band, he would gladly step aside (though this is suspect, because he's not the hurdle, Ian Gillan is). This is a dated reference now that Chris has passed, but you don't become the singer or keyboard player of Yes without some understanding that you're keeping a seat warm for Jon and/or Rick.