It's not unrealistic. This is the point and value of having competition. If at each layer of the system they won't stand for your beliefs that violence is okay, for example, then you'll have to create the infrastructure at each layer if you meet resistance at each layer. If they're serious enough and organized enough and the rest of society isn't willing to budge, and if the "offenders" aren't willing to reason and adapt their ways to not be violent - and to have open critical thinking dialogue, if they can be un-brainwashed from Trump et al's lies and the lies perpetuated by the duopoly - both sides harmful but the treasonous Republicans (that's not all of them) who are willing to incite violence is obviously the worst of the two - then friction and physical conflict will occur. This will either turn into a civil war, where both sides will need to create the infrastructure including arms and readying defense, or if these people can be reached and government policy be put in place to break apart the duopoly, re-enact things like the Fairness Doctrine so news legally must present both sides of the story to their viewers, along with the core policy proposals of Andrew Yang - Democracy Dollars, Journalism Dollars, Ranked Choice Voting, Freedom Dividend/UBI, etc - then the quality of life of everyone will improve, stress will go down, and dialogue and conversation will become more nuanced and diverse again, not merely the echo chambers of the two narratives of the duopoly that's been captured by industrial complexes and bad actors for many decades extracting as much from society as they can, suffocating everyone slowly but surely.

If you want the convenience afforded by the technology of private companies then you will have to follow their rules; if you show that you're racist while a guest in my house, I'm going to ask you to leave and if you refuse I'm going to call the police to have you removed.

Android does allow side-loading of apps, and Apple has the ability to prevent already installed apps from working. I think ultimately this is a question of whether you want your opponents/"enemies" - those who aren't aligned with you - to have access to the same level of technology as you: is it a good idea to level the playing field or to have an advantage? Arguably it's only good if the owners/controllers of that technology are on a more right/good path than the "other."

I assume in part Parler expected or wanted this deplatforming to happen as free marketing and to rally individuals to the platform, rallying the energy of people feeling persecuted - because enough of them were openly calling for violence and moderators weren't removing those comments/users from the platforms - but they don't seem to be having that sane/reasonable conversation to calm and quell the angry mob - and any "outsider" who attempts to reason with them gets ban/blocked, so it's inevitable it will spillover into the physical world. I feel and think it's good to slow this mob down however possible, even if it angers some of them more - they were already substantial open calls for violence, so it's not like deplatforming was the cause, the cause is Trump et al's lies and propaganda to incite violence; where's Trump ingenuine call for "law and order" after the Capitol was stormed? Answer: Trump told people to go to the Capitol.