you do not have to join. You will however have to pay a smaller service fee for the benefits of the contract anyway.

again, they absolutely protect the contractual compensation, and they do it well.

you're mixing contract enforcement with contract negotiations. Regionals - under the current business models and RLA - have a much weaker hand then they should.
mostly due to the business model. You can't go to a Delta pilot and say take these concessions or your flying is getting transferred to United. Until the RLA recognizes the outsourced business model regionals will always always always be subject to whipsawing one against the other. This only applies when there's a surplus of pilots ready to take the job. When staffing became an issue around 2015, it slowly switched from cheapest provider to who can actually staff the flying. We have about another 5-8 years to get our act together before the whipsaw is back in full effect. ALPA & IBT are not planning far enough ahead. When I was on the ALPA BOD in the 20 teens they didn't look far enough ahead then either. We did effect some change, I got defending the ATP rule passed as a PAC priority. We also got national to create a Regional Coordinator position in the presidents office so regional unions had direct access into the presidents office without getting lost trying to work through the bureaucracy. I left in 2016 and they've done very very little long term planning since. Mostly putting out fires... 67, single pilot, NAI, and stuff like that.

but to answer your question... you wouldn't want to work at any of these airlines without a union. You complain now? I can't imagine what you'd be doing without the union. I myself was never a big union person until I worked at an airline. Every employer prior to airlines was more than fair. This industry just doesn't work that way and managers are far far too willing to look to employee concessions to increase profits and make themselves look good.