Your good example is America West? From a pilot perspective, it's only worth it if you're looking back today. Since 2007 to about 2012, they had severe infighting, voted out ALPA, and got USAPA using pure majority numbers of East versus West. The SLI was done first and then a JCBA - that never came, because they didn't like the result of the underdog AWA getting the "windfall" on the East side. It changed ALPA Bylaws going forward and now that is why the JCBA is done first, and SLI second. West v East was a complete mess and the true synergies weren't recognzied until the AA merger. Today, sure West guys are flying widebodies. But the post merger years were terrible.
Republic Frontier/Lynx/MidEx was a complete mess, the only recent example so far of a major trying to merge with a regional. Agreed with your sentiment on that, it was done horribly and fell apart.
VX/AS. The planes are gone, one base is gone, and some routes disappeared. We still maintain slots and operate from the east coast cities VX did. They just don't all go to SF or LA anymore. Importantly - every single VX pilot maintained their job. The biggest complaint was losing the NY base and having to relocate or commute to California for this job. If we are to be honest, we can't ignore the result of the pandemic. Business flying is still very dead. It just hasn't come back. Most of VX's north east routes to DC, NY, Boston were business oriented. Few leisure travelers here. If Virgin existed standalone, it would have been forced to change its business model.
There are other mergers that were left out. NWA/Delta. And even that closed MEM, eventually closed CVG, parked 747s within a couple years, parked 777s eventually (mostly due to the pandemic). CO/UA is was able to keep pre-merger bases and hardly much change to the fleet model (Continental was simple already with 737, 757/767, and 777s). The SLIs for both these airlines went fairly well, all things considered.
Like you, I am optimistic. Hoping for the best for both sides