Really, I think Fr Hesse's view was a good-faith effort to explain the crisis. (Even though, now that Benedict XVI said that the new rite was part of the Latin rite, this means Fr Hesse's view was factually wrong. But his attempt, at the time, was the only logical solution....I'm not condoning his new rite ordination...I'm just saying, he's one of the few people who rightfully saw that the new rites were, on paper, invalid.)
Classic R&R/+ABL erred in trying to rectify/argue the validity of the new rite, and (practically speaking) ignored the licit/schismatic problem.
Sedes (for the most part) go to the opposite extreme and declare everything invalid, while not concentrating on the licitness issue.
Fr Wathen said that the new mass was certainly illicit (i.e. schismatic), but only the future Church could decide validity.
The error that +ABL/post-ABL sspx made, is to not take a stand on the illicitness of the new rites. Practically speaking, even if you argue the new rites *could be* valid, you must advise others to stay away, due to their being illegal. Canon Law tells us to treat them as invalid. This is the proper middle road.
Had +ABL taken such a stand, then sedevacantism may not have gone too far in the other direction (not theologically extreme, but humanly speaking, where they could not work with the sspx to build Tradition), as a reaction to the sspx's *apparent* lukewarmism. And +Fellay wouldn've never had the opportunity to take the sspx down the 'slippery slope' to which the over-concentration on validity led...which is the modern-day, new-sspx indult.
On the whole, sedevacantism did go extreme (in my opinion) but not in the realm of theology or doctrine, but only in human terms. But we're all human, and especially in a crisis, such can be expected and not condemned. +ABL went extreme too, but again, I can hardly blame his motives. +Fellay, on the other hand...