The richest player in SGM is right about it being an expensive snare for players with low resources, but I think the gold fighter deal is not that bad.
Essentially the SES is asking:
"Will you trade 5 chance relics for 1 certain-result relic?"
There are like 80 gold fighters in the pool. Every time one opens a gold relic, you have a 1 in 80 chance of getting the fighter you want (and the SES fighters aren't even in there yet). Odds are you will get a duplicate gold.
First, I'm flattered, but no, I am not the richest player, I'm just the most...frugal player; my 'hoard' is what the game has (potentially) given to all players that reach a minimum of 10% on all events and completed all tasks daily for 5 years (I've been playing longer, but hey, you don't start getting 10% in the first month, especially back then).
That aside, yes my post was a bit dramatic, but as you said the take away is that SES is a resource-sink and that for players with low reserves should not use without knowing what they are truly spending.
I agree, the SES for gold exclusives is not a trap. I mean, players sac 5 golds to evolve a gold to a diamond, so that is a cost players can already afford.
(My point still stands about Dupe-protection, if that was in place, there really wouldn't be as great a need for targeted acquisition options).
However, I think that much of the ire/backlash comes from not just the cost of the Diamond SES, but what the SES really is. I'm sure that many players voiced issue with the difficulty acquiring BP variants in the surveys. The 'problem' with timed exclusive-ness is the a balance of how long something stays exclusive; too short and you upset early adopters and it doesn't feel to special any more, too long and the 'have-not's get impatient and/or vocal about AOMO (Anger Of Missing Out). And I think the initial release schedule for BP variants in the general pool fell into the AOMO side when RNG is factored in. So, the SES drops to bridge the gap between when BPs end and when variants are added to the general population.
The Devs really need to be a bit more transparent in regards as to their reasoning and intentions when implementing a new way to acquire variants (outside of relics)...especially Diamonds. SGM is not the first or only gatcha game out there, and SGM users that have played other mobile games have developed different expectations when it comes to 'pity systems' and/or 'purchase-able' variants. Though, almost all mobile games start with RNG blind-box system and over time as rosters grow (cuz there is always an influx of new characters/variants) the players need a bit more agency in getting what they need/want. But I think, at least for SGM, is that when they rollout some new targeted system, they reveal it with their 'Dev goggles' on, meaning they already know the reasoning and intention of the new system and that the new targeted acquisition system is a supplement not a replacement for the Relic acquisition system. Players OTOH, see it through a slightly different prism (they want to replace the Relic system due to the frustrations of RNG).
I think that if the Devs put the info they share in Discord/socials/etc., after an update, regarding the reasoning and intentions of new features in the Update Notes (when published) it would save everyone a lot of heartache understanding and evaluating new features.
I love the rework of so many characters, especially BB, Raw Nerve, Tomb & Hellcat. Been using my Tomb since update and it's noticable how much better she plays. Being able to trigger her SA with a special is a game changer imo. 
