I agree with you on giving players a reason to play without forcing them, but where I disagree (respectfully) is on the concept of rank decay.
Most competitive games have rank decay to encourage continued participation, and it will keep those in the Top 100 engaged. Another added benefit is that it discourages RMT/wintrading (or at least make it more difficult). As such, I feel it's much needed for Feast ranked queue.
In regards to the lower tiers, (my opinion, which I assume is the same as yours) it's a matter of game design and whether-or-not the game mode is enjoyable to the general FFXIV player base. The caveat here is that game design (or game development in general) is difficult, and giving what players want is not always the best way of doing things, I'm sure the devs know this. Regardless, I think Feast must be developed into something that almost everyone within the FFXIV community will enjoy without relying heavily on providing what players want.
Feast IMO is NOT an enjoyable game mode to the general FFXIV community. It is stressful, it lacks DEPTH, and it is NOT easy to understand. For example, most players don't know that you win matches by picking up medals. Non-healers pick up Wolves Heart with no idea of what it does. There is no chat available for seasoned players to mentor newer players (except for custom matches). The concept of bursts, mitigations, and positioning are alien to most. Netcode is bad within this game... and it makes it worse for PvP. The culture in FFXIV seems to be more anti-PvP than it is to actually support the idea of improving it. The list goes on.
Because of this, Feast will continue lacking in participation unless there is some kind of reward or incentive to queue the game mode... I personally want to see it queued because it's FUN.
Feast needs some love. Until then, lets see what the new PvP game mode will provide. XD