To expand further on why gems being the huge “get” is a bad idea for this mode, I’d like to bring up a few other things:
The more gems you have and have spent, the less inherent value each gem has.
A starting player might be able to spend, for example, 95 gems and get a new troop or two out of it. A player a few weeks in probably already won’t get anything useful out of that level of expenditure. A player at midgame missing a few legendaries has a very very very slight chance of getting the one they want. A player at endgame missing a few mythics has a fraction of a fraction of a fraction of a percent chance to pull something useful, and a player at endgame missing no mythics just has a bit bigger buffer to not get screwed when they dump massive amounts of gems on the next exclusive mythic.
Losing a few gems any given week has far more of an impact than gaining them.
If I were to put out for example, even a couple hundred gems per week from my stash, I’d be below what I estimate as the “even chance” category for obtaining the next exclusive mythic. Gaining a couple hundred per week would hardly help me at all in the short term or the long term. Short term, I cap out how much I’m willing to pay on the next exclusive mythic, and adding a couple hundred gems to that wouldn’t even significantly alter my chance when I’m already spending 4k+ most of the time, and long term… well, general mythic drop rates is beyond abysmal, where the average price tag to pull a specific one randomly is now in the tens of thousands of gems. I can’t afford to lose any gems, and gaining more does me very little good unless it is in orders of magnitude.
Both of these factors considered, the cash price of gems is quite frankly absurd to all but the beginner level looking to scratch out some starting troops.
The value of a gem is not equal at all levels and is an integral part in both unlocking content and progressing in the game. Attempting to even get in the ballpark of a full collection, not even counting mythics, using glory keys and gold keys alone is a hellish affair that would likely wear on even the most patient of players.
In the past, the economy has been rebalanced around what is possible by the top earners.
I’m not just talking about the guild task retune, but also the removal of legendaries from Glory Packs and putting them as event key only for one week, nowhere for four weeks, then making the only way to get them for the following ~5 months after is the abysmal common drop pool. Any type of currency can never be claimed as extra so long as there is a threat that the bar will be moved again.
Putting those points together, if you are one of the people sitting back and salivating at the prospect of all those extra gems for reaching one of the top spots, chances are, you probably won’t ever be getting one of those spots. If you are in range to recoup, fully upgrading sentinels with gems is pretty much going to be a wash for ranks 3-10, and 11-20 would already be taking such a big hit that they probably wouldn’t consider fully upgrading on following weeks.
In the end, troop tokens make far more sense because the odd amount of guild wars troop payouts and the inability to get them from anywhere else means that, bare minimum, you’d have them stuck at whatever ascension you obtained them at for least six weeks, presumably forever at some point when their rotation ends. Being able to pick up content you weren’t around for initially is also pretty essential for new people coming into the game.
A reduced sentinel cost with no chance of recouping the gems means that people in the top guilds with 20k+ gem stashes can funnel some of those earnings into guild wars for a stat advantage if they want to (so they have an easier time using a more homogenous color team and therefore get more bonus points and higher ultimate rank). Underdogs can funnel some of those gems here and there if they feel they need it and it will give them a better chance (probably still wont, but it would be more considerable if the costs were more reasonable). Those that are beyond the reach of reliably targeting mythics but can still reliably get every legendary might even consider it, but only if the costs were more reasonable. But at least the ultimate reward wont be more gems to funnel back into guild wars in a cyclical manner.
Plus, I’m almost certain far more people would be willing to just straight buy troop tokens to ascend x or y troop to the next level a bit early than would buy gems and then funnel them into sentinel bonuses to gain a slight advantage (or be at slightly less of a disadvantage, depending) for thirty personal battles per week that contribute only 3% of the total score that will eventually determine your guild’s ranking. When you spell it out like that, it almost sounds absurd that anyone would even consider taking that path, especially when 340 gems is $20-30 “worth” of gems.