By 1988, Archbishop Lefebvre considered all Novus Ordo ordinations to be doubtful.  His judgement did not arise from problems with the matter or the new form, but with the intention.

The intention "to do what the Church does" is always presumed if everything seems alright.  The Church does not try to look into the bishop's soul.  Positive doubt arises when the bishop says something which contradicts what the Church has defined about the sacrament.

When a certain bishop publicly taught that Confirmation does not give the Holy Ghost, Archbishop Lefebvre declared all of his following confirmations to be certainly invalid by defect of intention.  The bishop himself had manifested by his teaching that he did not intend to administer the Holy Ghost in Confirmation.

In the earlier years of the crisis, some bishops began teaching new concepts of the priesthood, including some things that directly contradict what the Church teaches about the priesthood.  Archbishop Lefebvre publicly questioned if it was possible for these specific bishops to ordain priests anymore.  His investigations into Novus Ordo priests coming to the SSPX will have included this consideration.

At least by 1988, these new concepts of the priesthood were no longer isolated in occasional statements by the most progressive bishops.  They had become generally accepted.  Most bishops will have had a seminary under his responsibility where these ideas were being taught to the seminarians.  Any bishop who didn't speak out for the true nature of the priesthood while the new errors were triumphant around him was at least suspicious for his silence.  These considerations led the Archbishop to recognize a situation of "universal doubtful intention" regarding ordinations by bishops un-separated from Modernist Rome.

Can someone in the SSPX furnish a quote after '88 displaying a retreat from this attitude?  Will any of them explain why they have departed from their founder's spirit?

It's entirely certain that there's positive doubt about the Novus Ordo Rite of Ordination.  Period.  And there's no reason NOT to perform conditionals on all Novus Ordo presbyters ... except that the SSPX are playing politics and subjecting the faithful to invalid Sacraments from fake priests who go around simulating Masses.

This is all explained in the "sspx fake priests" thread.  Pope Pius XII defined the form of ordinations/consecrations.  The new rites differ from what Pius XII defined.  Ergo, there's doubt.

"One can say a Catholic sacrament involves five elements : Minister, Intention, Matter and Form are essential for validity, the Rite surrounding the Form can be important for validity by its sudden or gradual bearing on the Minister's Intention. For priestly Orders, the Minister has to be a validly consecrated bishop ; the Intention is his sacramental (not moral) intention, in ordaining, to do what the Church does ; the Matter is his laying of both hands on the head of the man to be ordained (women cannot be validly ordained to the priesthood of Christ) ; the Form is the crucial formula or series of words in the rite which express the conferring of the priesthood ; the Rite is all the other words surrounding that Form, and prescribed in the ceremonial rite of Ordination.

 In a new rite Ordination, if both hands are laid on the head, the Matter is no problem. The new Form in Latin is, if anything, stronger for validity than the old Form in Latin (by the « et » instead of an « ut »), but vernacular translations need to be checked to make sure that they clearly express the grace of the priesthood to be conferred. Most of them surely do." Bishop Williamson, 2009, EC 121

I posted Bishop Williamson's words to help put to rest the Diamond Brother's nonsense about the change of the word 'ut'.

With regards to your proposal, I understand what you are saying, however, canonically, a positive doubt - in both cases, must first be established. Otherwise, according to Church Law, to re-ordain without an established positive doubt, is to commit a sacrileges act. And rightfully so because you are calling into question an official Rite of the Catholic Church. I'm not arguing against Re-ordinations - at times they are absolutely necessary and the Church allows for this - but what I am high-lighting is that from a very early stage, if not the beginning, the SSPX practiced a cautious policy on only re-ordaining where a positive doubt existed; that they did not automatically do so simply because a priest had been ordained via the New Rite - of which they formerly ruled was indeed valid.  This is the same prudent policy that the SSPX exercises today. As Catholics, and as traditionalists who follow and maintain Archbishop Lefebvre's 'Recognise and Resist' position, we are obliged to follow Church law as it has always been instituted. We cannot enact new laws out of personal fear and assumptions. We must be careful that we do not over-step the line for the good of the over-all Church. With regards to the essential elements, it had been concluded by the early leadership of the SSPX that they is no ambiguity in the New Rite of Ordination. If you believe me to be wrong on this point, please submit your evidence for us to consider. Thank you.