@OP - blanket statements just weaken credibility; you can make a case that Engineers didn't provide 'enough' diversity, or too much, or too little vs what you wished. All fair positions to take as that is simply an opinion.
But saying Engineers didn't increase diversity is factually incorrect and skews your OP by essentially denying the obvious -->
Pre-Engineers, whether you liked the state of those ships or not, whatever your opinion good or bad, it is a factual statement to say those ships were pure, 100% identical clones of every other ship of that class owned by every other player in ED universe. The only distinction or diversity was what 'build' a player used, otherwise every component of that 'build', every ship class was 100% identical.
With Engineers, whether you think the modules suck, are great, not implemented well, whatever - it's just a statement of fact that ships are now differentiated not just by the component choice (e.g. build variety of that ship class), but the individual parameters of the components themselves.
It's quite true to say for some it may very well be the FOTM or 'most popular' choice of one specific derivative of that engineered/custom component - but the very fact that choice exists means ships are more diverse simply because in addition to class and build, we now have class, build, and customized components decisions.
Just because many choose the same choice does not mean diversity does not exist; it simply means within that increased selection of possibilities, people are gravitating towards a common choice, but invariably as in all human endeavors, some people don't go with the flow. Pre-Engineers, there was literally no choice at the component level; with Engineers there is.
Your issue seems more to be that you don't like people making generally the same customization selection, which is a different point than denying those additional choices exist at all.