[mst]

What kind of use has a hood on a 50-250 or 16-50?  

To each his own, I made it a habit to use them as flare protection... and occasionally physical protector, which they manage to be much better than a front filter

Still: it would be nice to leave the decision if a hood is a useful or useless piece of plastic to all buyers and just include it for free, like with most other lenses. Instead, they expect those who want to use one to pay another 50 EUR for two peaces of cheap plastic.

Regarding the mount construction: you maybe haven't opened many current lenses, did you? Screws going into plastic constructions is the rule, not the exception. Nikon does that on many lenses (including many of the modern AF-S primes and zooms, also most of the Z lenses), same with Canon (including L zooms and primes) and Sony. You will rarely find all metal constructions inside, occasionally though metal thread inserts in the plastic frames, that don't wear out too easily. However, wear is something that depends on what kind of plastic material is used... and those screws are not meant to be removed and reattached hundreds of times anyway.
A heavily used or even mistreated lens will show signs of wear over time, that's true for cheap kit zooms as it is for Canon L zooms. Scratching when zooming is usually a sure sign that the lens was used in an environment it wasn't intended or designed to be used in... on a beach, in a dusty place (like right next to a rallye track or a teardown site), things like that. I still find the construction of the 10-30 VR solid... mine is still working like new, at least

In any case: why even bother about dust in there? It won't show up in any image...