The industry has made a market segment for this/largely ignored it, 'HEDT'. I'd say this is evidence to your point, GPUs being the motivator. I'm starting to think I'm the only one still using add-in NICs, sound cards, or bifurcation.
The ATX spec allows for 7 or fewer physical slots. That's not what's letting me down!
How many lanes are actually available and their division changes over time. Everything from USB to SATA has internally been moving to PCI-e, there's more 'internal consumption', too.
Gen5 is kind of a victim of success. Each individual lane is very fast, but providing them to a fair number of add-in cards/devices isn't practical. Simply physically, but also bandwidth between lanes.
Pretty much all of gen4 is nice, those systems are perfectly viable. More lanes that were slower offered me more utility. It's multi-faceted, I overstate it too. I like hyperbole :)
