Some worry that consent-based federation would lead to isolation, but this doesn't need to be true. For the end user, nothing at all would change; it would be just as easy or hard to join an instance as it is now, minus the funneling into a centralized, poorly-moderated vanilla flagship.
And what about new instance spinups? Their prospects could actually improve from the current status quo, if an allow-list fediverse was structured into instance alliances as described in the fedifam concept. New intra-fam spinups would automatically federate: https://kolektiva.social/@ophiocephalic/110793531238090472
Fedifams could then form trust-treaties with other fedifams, smoothing federation out from the fam into the broader fediverse in whatever manner of comfort or caution is preferred (such as limited or probationary federation): https://kolektiva.social/@ophiocephalic/110985194948458666