I don't recall citing 7 footers as the source of incompetence (kidding). I like 7 footers - lucky thing since the last of the pool rooms here can't find a space to get restarted. As the more informed have stated on numerous occasion, the 'dead bread for a little bit of tough action' crowd (my words) is probably the reason for the decline of the American pool star.
To me pocket size is the culprit simply because pool is primarily marksmanship. Note, I'm not ignoring cue ball here. Tight lines make equally tight cue ball. The more adept you become at pocketing, the more mental resources you can devote to cue ball control. American attitude used to be enough until the Pinoy Nation got involved. Then you had to lose the BS and learn to play real winning licks. America never quite got the hang of that and before that even gets started turning, the rest of the world got in gear with the precision technique(s) to where the best are virtually freewheeling those precision lines. Tight pockets are vital to pool. They shows you the work in no uncertain terms. Granted, extreme equipment belongs in pool conservatories but the need for both is vital.