Feels like much ado about nothing to me. My guild doesn't have a single R7 TC but we weren't filling P3 platoons anyway. If we were, losing out on 1/6 isn't going to significantly change the landscape on Tatooine. Also this is a single character, occupying a single slot, and it's clear they're doing this as a way to allow for 5th to be used in the Reva mission. Of the people in my guild that are completing the Reva mission, they've all done it consistently with 0/6 platoons filled.
There are maybe a few edge cases where this change might result in one less star for a very small handful of guilds for a single TB. And that's making the very large assumption that no one in those guilds could manage to get Chieftain to R7 within a week - which just doesn't seem likely considering 50 people in each of those guilds.
The assertion that Chieftain is useless is outright bunk and false. In 3v3 he's part of a very strong team with BFSoJ. In 5v5 tuskens are a fantastic counter to several TM based teams. In conquest there's no better way to deal with an imperial troopers team. When tusken GCs come back around someone will be grateful for the extra relics. So putting gear on him is nothing like being required to put relics on Ugnaught or something like that.
Sure, it would have been nice to have a little more notice - but for the people who this actually affects, it was a nice gesture on CGs part to do this sooner rather than later - it means one more Reva shard for people that were filling that platoon. It seems people are immediately jumping to the conclusion that this is going to be a regular thing done on a whim by CG and I think it's clear that that isn't the case given the obvious intent of this particular change.
Don't get me wrong - I'm all for having more notice about upcoming changes. This discussion just feels very knee-jerky and some very strong assumptions are being made based on one minor change that was made for the benefit of everyone engaging with that particular content.