The implications of sedeplenism (which I assume you are conflating with R&Rism, since there are plenty of sedeplenists [most, in fact] who accept the conciliar magisterium) are far more dangerous and complicated than those of sedevacantism.
R&Rism is simply not supported by Catholic theologians. The usual tracts about resisting the head (St. Bellarmine) and refusing an immoral command were never written with ecuмenical councils, universal liturgies or codes of canon law in mind! These quotes are contextual with private commands, and at best maybe very, very local "laws" which would not in any way be confused with universal ones or anything which would touch on the universal disciplines of the Church, which are infallible. No theologian can ever be found speculating on whether or not the Church could promulgate harmful laws and liturgies. Such a belief is contrary to Her nature, and was never even entertained-- and keep in mind that theologians entertain A LOT of things that they don't privately think could happen, but they do not entertain that which would violate Catholic doctrine, which this idea does.
It runs roughshod over the nature and purpose of the Church. We are left with a Church that is essentially powerless to protect Her children from error, in fact, even approves it and recommends it! The Church becomes a trailer trash single mom, feeding the little one's McDonalds day in and day out. Unreliable and whorish.
This can only be solved with the (apt) Conciliar/Catholic Church distinction (all that which is un-Catholic is the product of the Conciliar, not Catholic Church) which most R&Rists are keen to make, but this actually brings them closer to sedevacantism, because by logical necessity the putative pope who has approved these doctrines for the Conciliar Church has lost his office by not only belonging to but by solely officiating a false religion! Shockingly, some will not admit that this poses any difficulty. By their own admission, the pope is not Catholic-- and that's OK! Hopefully critical thinkers will realize that accepting the non-Catholic pope is actually expressing the very same Frankenchurch ecuмenical errors of Vatican II that they claim to reject, incorporating those who do not profess the Catholic faith into the Church of Christ. Logically, there is no impediment to the Dalai Lama becoming pope, or to having a schismatic Orthodox Patriarch become pope if one accepts that the Conciliar Pope can also be the Catholic pope.
And of course, if one fails to make the distinction between conciliar and Catholic (as the NSSPX seems wont to do) then one is left with much worse than a contradiction, but with an objective blasphemy (without saying that those, confused, who believe it, are guilty of blasphemy). The Church is an ugly mistress, trying to please all the different men of the world by bending to their passing whims, trying to impress them by complimenting them on their base and sinful tendencies. It is NOT the Bride of Christ, it is the bride of the world, which is to say it is the bride of Satan!
Of course, the reason for holding to R&Rism is typically lineated as preserving the Church's visibility. But in attempting to "preserve" it, it absolutely annihilates it. The Church is a visible unity of faith. By accepting non-Catholics as not only members but representatives and even the visible head of Her, that unity is utterly destroyed. While R&Rism typically balks at SVism because it makes the Church "invisible," R&Rism results in a Church which is all TOO visible: in fact, it's everywhere! Your Jєωιѕн mailman represents it, your protestant co-worker represents it, your atheist sister-in-law is a part of it, etc.
Of course, SVism doesn't actually make the Church invisible-- at least, SVists don't think it does! They wouldn't believe it if they did, and wouldn't spend so much time explaining how it doesn't. So far as I can tell, as long as the Church is visible to SOME, it is sufficiently visible. The Church at Pentecost is essentially the Church of today. It was "invisible" to the vast majority of the world. But the Church began at Pentecost with all of Her marks. She did not acquire the mark of visibility only once She had physically spread throughout the world. She was visible from the instant She began, and the scope of Her visibility has grown and diminished accordingly, but She is visible nevertheless.
SVism isn't without it's proverbial hiccups, but they are more practical than anything else. They certainly do not boldly challenge the nature of the Church in the way that R&Rism eventually will do.