We tested this with FS on different worlds that contained several of the same members, and the FS that allowed cross trades often had normal trades languishing for days whereas the FS with a rule never had trade requests up for more than a few hours before being fulfilled.
I do have to wonder how much your bias against cross-trades and that of your fellowship impact this. One of my fellowships has not restricted cross-tier trades (even before the changes) and we have never experienced the issue you mentioned. We have had the odd grumble but people aren't stopping trading and we have had no complaints since the change.
Cross trades flood the trader
Some players like me ignore the trader to avoid the hassle instead of helping fulfill trade requests.
Newer players have their trades taken less frequently if at all
Smaller players quit more regularly and/or spend less money
Elvenar profits are lower
I think your over exaggerating this. I am sure there is a small effect of this but small, pretty sure other factors are much larger in impacting retention/profit. To be honest, with so many active neighbours people don't always check the trader regardless of what trades are posted. The small towns me and my family members play have zero trouble getting trades taken regardless of the fellowship, world, etc.
Interestingly the fellowships that restrict cross-tier trades are more problematic for small towns from my experience. As an experienced player I still get my balance of goods production not quite correct on a small town and need to occasionally readjust, but can't or takes longer to adjust production when a fellowship restricts trades. New players have less clue about production and are more likely to get the mix wrong and need some quick adjustments. Far easier to help them while they learn then to ignore them or worse yet blow up at them (have heard of some people experiencing this).
The forum is a small representation of players and I think it is fairly obvious the main player base has less issue with the trades otherwise there would be less trades. They also are still very willing to grab trades of small players. If a person really doesn't like cross trades the best thing they can do is gravitate to a similar fellowship and use the filters.
Considering inno's aversion to limiting trades I do suspect they won't change this. Worse yet haven't people learnt to be extremely careful what they ask for? Do you really think that if you convince inno to mess with cross-tier trades that they wont also mess with them for Sentient and Ascended goods? If people aren't concerned about that they obviously haven't played this game for very long.