Television. Online and paper news. Literally beaming messages directly to the cellphones of every American.
So long as you think there's at least one person who he reaches by social media that he wouldn't reach with traditional media, it seems you are obligated to conclude that banning him from social media would reduce his reach, or in other words, trouble him getting his message out.
I'm sure he could still easily get huge messages out, but would have much more trouble getting out minor messages of the kind he uses Twitter for.
And yes, this would reduce his reach. So would a newspaper or TV channel not publishing something he said, and yet we'd be incredibly weirded out if newspapers and TV channels were required to publish every mundane thing the President sends their way.
You're assuming that the convention of neutrality in these programs would be upheld, but it's just a convention and iOS already violated it a long time ago in the mobile space. It was after all, once a convention that tech platforms didn't attempt to decide the correctness of what people said, on the grounds that they were just platforms and not publishers.
Additionally the issue here isn't specific to some vague category of tech companies. This is happening because of activist employees like those at Facebook. They can and do crop up anywhere, they can and do impose very nasty demands. It's not merely about de-platforming their opponents, but also forcibly platforming their allies.
For example, Kickstarter's employees unionised in order to force Kickstarter to keep supporting a fundraising project that was openly advocating violence against conservatives. This broke all the rules of the site, which is why it got taken down, but such details didn't matter to Kickstarter's activist workforce - who via unionisation were successful in their goal of converting Kickstarter into a fundraising platform for leftist violence.
There's a clear pattern in which hard left employees seek to both erase their enemies and also make their allies immune to the usual rules. This behaviour is tribalistic and destructive. Judging from history it ends in total cultural cold war, and possibly even hot war. At some point people have to say enough is enough, and that not every company has to get into every fight because a few employees walked out.
I guess tech executives are hamstrung by Californian law and can't simply fire employees who don't turn up to work. This is just one more factor making Silicon Valley unattractive as a place to set up new businesses: set up shop in California and before you know it, you'll have ended up with some hyper-aggressive leftist employees trying to take over your company.
The problem here is in letting a handful of social media companies have an outsized impact in our democracy. That's not worth booting Trump off Twitter.