It's a self-fulfilling prophecy. Players who multibox who claim they'd quit if they can't multibox do so because they are so accustomed to multiboxing. If SE had policed multiboxing from the start then those players wouldn't be accustomed to having that advantage over other players, and the community would be healthier as a consequence.
FFXI isn't the same game as it was when we had to wait 3 hours for NMs to pop, so that's not an excuse to multibox. Everyone is busy; a person who decides their time is worth more than fair play isn't some sort of visionary who is better at being an adult than everyone else.
Multibox players contribute to the game financially, but there's no way to assert for certain that this is actually a good trade as the very existence of multiboxing destroys community play. There are plenty of people in this game who lament being unable to access content, or having to pay a multibox army run by one person to get clears or acquire gear. How many people have quit because they can't do what they want to do? How many people have quit because they're sick of being in an alliance which is half automated? We don't know. There's no way to say if the multiboxers who caused this damage are compensating for it economically; however, we can be absolutely certain that they can't compensate for the reduction in the size of the community. Bots aren't people.
Trust are intentionally limited. Whether they are a good thing for the game or not is another debate; they aren't, however, directly comparable to a person multiboxing. Everyone has access to trust - not everyone has access to a perfectly customisable party of bots. And of course, a multiboxer can clear tough content. Generally speaking, you can't do that with trust. There are exceptions, of course, but they are in the minority.
And the elephant in the room is that it's not possible to multibox without cheating. I've tried to run two characters on two different machines - perhaps some people can do it effectively, but I really don't see how. It's almost impossible, and it's exhausting. So every single person who's got a small army on /follow behind them is cheating to some degree; they either have means of transferring commands to their other characters, or those characters are straight up bots. That's a massive advantage they have over other people who are using trusts, or even partying with other people - it's intrinsically unfair. Advocating a change in the game and justifying it by stating that it'll help someone cheat more effectively is absurd.
I have some sympathy for the people who've grown dependent on multibox cheating because, for them, returning to playing on the same level as everyone else would be a massive jump in the difficulty of the game. But that's not a reason to allow it to continue. I'm very tired of people asking if they can "bring their alts" to everything and having bots whirring around in the background while I'm trying to enjoy myself. They certainly aren't enhancing my game experience. There have been many days when I've just not wanted to log on because I'll have to deal with multiboxers and bots in almost every zone I go to.
So yes please to reducing knockback spam, but let's do it for legitimate players and not for people who want to cheat with bot armies, eh?