> Scrum is iterated waterfall.
No, it isn't. Iterated waterfall is at least as old as the first paper discussing waterfall, but while scrum mandates interations, it doesn't mandate much about how work is done in the iterations, and specifically does not mandate the process steps associated with waterfall; further, it emphatically rejects the role separations and handoffs associated with waterfall during the iterations.
> munching through backlog items fed to them by product managers.
That's...not actually Scrum, as it implies that either the role of Product Owner is taken by a PM outside of the Scrum Team or that that the Scrum Team is not self-organizing, either of which is a significant (even if common) deviation from Scrum.
> the blinkers of "sprints" encourage growth of tech debt because nobody has an eye on the future
Tech debt should manifest in reduced velocity which should be noticed, taken as a signal of a process defect, and addressed in the Scrub Team’s various inspection and process adjustment points.
OTOH, if the Scrum Team is properly cross-functional and self-organizing instead of having a non-team-member imposed as Product Owner, then including appropriate restarting as components of completion of relevant backlog items shouldn't be a problem.