1. Management isn't going to agree to it. They like the whipsaw and flexibility. They hold the keys to a merger.
The airline industry is only one crash away from this paradigm shift. Once the public realizes that management is hiring highly inexperienced pilots to fly the public to save money, there's no number of airline name changes that will save them.
Comair 3272, 1997
Air Midwest 5481, 2003
Pinnacle 3701, 2004
Comair 5191, 2006
Colgan 3407, 2009
None of these crashes "shifted the paradigm". (Nice try to use a big word to sound smart). Colgan 3407 brought us the 1500 hour and an ATP rule, greatly increasing experience at the regionals, negating your experience argument.

2. The mainline group isn't going to take concessions to get you on board. Nor should they.
Isn't the whole point that Horizon is flying Alaska's route? Also I have no interest in going to Alaska--especially where I'm at now.
The point is that Alaska mainline pilots could take the flying back without the Horizon pilots is they wanted to take concessions. What are the Horizon (or any other regional) pilots bringing to the table. None of the regionals "own" their flying, and that's a significant distinction. As to your second point, good for you?

3. ALPA will fight you on it. Reference the showdown at the 2000 ALPA BOD meeting. (Were you even born yet?)
It's now 2021 and we're now starting to see the fruits of these "regional airlines". I think in retrospect B-scale wages would have been smarter, but I could be wrong.
You're wrong. Since you have no idea what I'm talking about, let me give you a little history lesson. In 2000, at the ALPA Board of Directors meeting, a rep from ASA brought a motion to the floor to establish a PID (Policy Implementation Date) which would trigger a single carrier petition, and a 3 way merger of Delta, Comair, and ASA. The Delta Pilots vehemently opposed this, and the Group A pilots hijacked it through floor amendments, then referred it to a committee and killed it. It was never brought up again. The RJDC lawsuit against ALPA, which drug on for years, was the response from the Comair and ASA pilots. That was eventually settled for pennies on the dollar, in exchange for "a seat at the table" in future scope negotiations. As for the B-scale, pilots who have been doing this a lot longer than both of us fought hard to eliminate it, and it would be incredibly selfish and foolish to bring it back now. You would get zero support on that, outside of the regional pilots who would be getting a windfall. The bad precedent would ripple across the industry as mainline management demands B-Scales. Allegiant tried it in 2020 to keep the furloughees on property. We told them to pound sand. It was the right thing to do.

4. It's a windfall for the regional, with little in return for the mainline. Difficult to negotiate a deal there.
There's actually a huge return for the "mainline" pilots.
Actually, there isn't. Alaska Air own the flying. You own nothing. You're a subcontractor. If the mainline pilots wanted to give concessions for scope, to lock down that flying, it would have nothing to do with Horizon. (It would take massive concessions though). It was incredibly short sighted of past MECs not to secure scope in that legacy contract. What is Horizon going to bring to that table to contribute? You own nothing.

5. The senior pilots at the regional won't accept a staple. The junior pilots at the mainline won't accept an SLI.
Virgin was technically a regional airline and they got an SLI? If junior pilots could flow back to the "regional" instead of being furloughed they'd accept it.
Virgin was technically a regional airline? Now you've lost all credibility. Virgin was definitely a mainline carrier. They flew their own code. Do you even understand what that means? They got an SLI because they were on the same level as AS, and a "merger of equals", despite the actual size difference. As for flowbacks, go paruse the AA forums. Flowbacks are popular when times are bad, they die out when times are good. It was also tried in the early 2000s. They were terrible employees when they flowed back, and management hated the program. It increased costs at the regional, and made for major training events. All it did was **** everyone off. Never happen again.