To really understand and answer this question, I would have to know about how legendary is going to change, if at all, next wipe.
The proposal is to make it so a 5 man can basically conquer most of arctic. Is That Not Already The Case??? The post references 1 healer, 1 Tank, 3 DPS/Utility. What % of arctic could a legend healer, legend barb, and 3 legend hitters take down currently? If i had to guess, I would say that that 5 man group could do 85-90% of the game and load tons of elite gear. They could plow through most mid/upper level older zones (i.e. storms, ft, dko, sanction, cv, luni, bluff, spire, etc etc etc). There are only a handful of fights that i am aware of that they probably could not do.
In my opinion, you immortals have already achieved what you are trying to accomplish - make the game playable for clans/groups of friends that don't always have a 10 man on. You did this through legendary.
If I understand the proposed changes, zones would become "easier" to suit the 5 man change, but yet a 10 man could still go steamroll the zones. So a 10 man would plow through zones even Easier/Quicker than they are now???
Finally, I agree 100% with Remi's post. I can tell you what would happen with RISE clan. People would be kicked out of the clan and an even more elitest attitude would happen. That is sad, but I would anticipate be the case in most top-tier clans. I think the current setup with legends allows a small group to have fun and do a ton of stuff in the game, but the current challenge in arctic is developing your clan so you have 10 qualified chars to do the final arctic fights.