I think people who feel this is controversial are missing the entire point of federation and should consider going to a platform that doesn't use it.
Nobody is 'mandating [an echo chamber] for everyone else' by defederating from a different instance, as many other instances are open registration. The largest problem here is that, in my opinion, the design is not well suited for overlarge instances such as lemmy.world or sh.itjust.works. We should all be on reasonably small instances that can smoothly choose who to federate/defederate and thus impact only a group of likeminded people. People with differing opinions can then just go to a different instance if they disagree. This is quite a democratic approach to problems like this, as it allows people who feel strongly about these things to 'shop around' for an instance that suits their needs and which will react favourably to further recommendations. If particular instances start hosting particularly disgusting opinions, they'll see a democratic process wherein a large plurality of instances all defederate from them.
In other words, you are seeing it as "defederation allows person X to determine what person Y can read" when in fact it should be "all people who feel the same as X are welcome on server lemmy.x". This problem is perpetuated not by people wanting instances that suit their needs, but by having a few specific very large instances that did not clearly lay out their philosophies (no fault of theirs I think, we're all learning this for the first time). They can no longer adapt with any agility due to a very heterogeneous and large user base.
On another note:
Has humanity really developed into a situation where words and thoughts are more hurtful than sticks and stones?
You really should study the lead-up to world war 2 if you think platforming dangerous beliefs is a simple matter of "words will never hurt me". I don't intend this as a 'gotcha' or anything, it's both fascinating and disturbing, and something every human should understand. The argument of 'we should at least let these fringe weirdos say their piece, what harm could it have' is, without exaggeration, how we wound up with ww2.