I believe that Bach would have appreciated Hanon, at least as beginner exercises.
Here is my evidence: Johann Nikolaus Forkel's famous biography of the master includes the following account of J.S. Bach's beginner keyboard teaching, which Forkel derived from personal communication with Bach's sons Wilhelm Friedemann and Carl Philipp Emanuel, among others:
[Chapter VII, 4th paragraph]
"The first thing he did was to teach his students his particular mode of playing the instrument... To this end, he required them to practice, for several months, nothing but single exercises (Sätze) for all the fingers of both hands, giving constant attention to clear and clean touch. From such practicing, for a few months, no one escaped. Bach himself held the firm opinion that it ought to continue for at least six to twelve months. But if he found that anyone, after some months of practice, began to lose patience, he was so obliging as to write little connected pieces (Stücke), in which such exercises were combined together. Of this kind are the six little Preludes for beginners, and still more the fifteen two-part Inventions, both of which Bach wrote during the lesson for a particular pupil and afterwards improved into beautiful and expressive compositions (Kunstwerke). Besides this finger practice, either in regular exercises or in little pieces (kleine Stücke) composed for the purpose, Bach introduced his pupils to the use of the various ornaments (Manieren) in both hands."
Single exercises for all the fingers of both hands, giving constant attention to clear and clean touch, for beginners -- sounds like Hanon, does it not?
I am confident that nobody, not even WF or CPE, EVER said that those "single pieces for all the fingers of both hands" were attractive or the least bit memorable in any way. Combined, elevated, and recomposed into such things as Little Preludes and Two-Part Inventions, it's a different story. But those first all-unison exercises, from which "no one escaped," and which clearly caused many to "begin to lose patience," must have been a bit on the boring and repetitive side.
And Johann Sebastian kept the poor little devils at it for months on end! The sadistic bastard!
Of course, Bach was a genius. But how much 'genius' can be put into music written for students at such an early stage that Hanon requires their full concentration?
Those earliest exercises cannot possibly have been much more interesting, attractive, or useful than Hanon's. If they were some sort of miracle of composition, Bach's sons would have remembered them all their lives.
Therefore, I think that if Bach were shown a copy of Hanon, he would have said, "Dass ist die Sort of Schtuff I giff to mein beginnen Studenten."