I also doubt that this Sentinel feature will be able to make some dollars (like fees to change Hero’s class, is there really players using it?)…
We are already saving our gems for the Legendary and Mythic troops “hunts” so I don’t think players will sacrifice them for a small stat boost (the best one could be +3 Magic and even that is meh)…
It is also a gamble for devs because they are going to give a lot of gems: 41,400 gems for the Guilds ranked 1 to 1000, and you have to multiply by the number of members by guild which I don’t know (in the case that the 1000 guilds are full it means 1,242,000 gems).
Moreover, it means that they are going to give gems to players so they will not need to buy them for the sentinel bonus: they will better give gem/event/guild/VIP keys to avoid that .
If devs want to make dollars, you can read this idea of mine ;-):