I was sort of in your shoes a few years ago, and did indeed end up switching; difference is that I switched TO Alaska, rather than AWAY FROM.
I'll say this (er, these) about your predicament:
1. Yes, loyalty is worth less now than before. With the mega-mergers we've seen this decade, basically any airline can connect basically any two major points at basically the same price with basically the same mediocre service and reliability. But loyalty is not COMPLETELY worthless. It does still have some perks, and the perks in the AS program are, in my opinion and in the opinions of some other travelers, worth more than the perks offered by the bigger players.
2. That was a good segue- other programs that you might have the opportunity to join are likely to be somewhat less (or even far less) rewarding. Admittedly, it doesn't sound like you care to use your miles. But Alaska is still awarding buckets of them for actually flying on their planes. And, global partners (at least, the ones that Alaska has bothered to enable award redemptions on) are still a very strong benefit of the Alaska program.
3. You've provided a good amount of info about your travel patterns; they're basically random. We don't know what your preferred departure point is, but I'm assuming it's NOT LAX or SFO, since you said you'd have to trek there. That means you're not hub-captive. You can do whatever you want.
4. All that said, I would say that your choice should maybe be between the following two options:
a. Become a free agent. Fly whatever suits you best at that time. Maybe it's United today (oh help us all). Maybe it's Allegiant next week, and the week after, Delta. Pick what matters to you: price? convenience? aircraft type? You choose.
b. Stick with Alaska, because it's not like the grass is especially greener anywhere else. Especially if you can get to MVPG75k... that comes with a big honkin' mileage bonus.
I hope that has helped.
Let us know what you decide!