My only comment on your question about why some think regionals will be gone soon is that who knows what will happen, but it does appear that things will change. At a minimum, they will shrink and consolidate. Maybe some go under, maybe some get bought out, maybe some get brought under the operating certificate of a legacy. At this point, it's all speculation.
But to answer you question on why older folks late to the party tend to stay at a regional, historically. As you read earlier, aviation is two steps forward and one step back... sometimes two steps back. Historically, you will build seniority faster at a regional and subsequently, a better quality of life. Remember me saying older folks tend to find ways to improve quality of life and not start back at the bottom somewhere? For those who are happy with a comfortable salary, $150k+ (even more now) and are okay with the regional business model, tend to accept the tradeoff versus what a major/legacy might offer such as potentially better work rules, potentially shorter workdays and potentially fewer workdays a month along with much higher pay, of course. And some, prefer to live in the base that a regional offers, that mainline carriers do not. But don't ever assume a particular base will always be there. Regionals close/open/grow/shrink/shift bases with some regularity, so keep that in mind if you happen to currently live near a regional base and are using that for thought.
Here is the thread that someone created to list all the domiciles for each carrier... however, the link does not appear to be working for me at the moment. (the link in the thread is on page 1)
Major & Regional Domicile Map