You'd also have to ignore the fact that Relic Weapons were released with Dynamis for every job and remained the best (or close) weapons for something like 9 years, with the only competition being Mythic and Empyrean weapons generally. They were the only pieces of non-situational gear in the game, at least partially because the game is treats weapon slots differently from others and assigns penalties for gearswapping them. They are designed to be permanent pieces, unlike all pieces of armor.
However, there are two big problems for SE that they tried to solve this way:
1) Buffs play a huge role in our damage output these days, so it is difficult to even out the damage potential of different players without adjusting weaponry.
2) The commonness of RMEs made a two-tier system, where RME holders could do really dramatically more damage than non-RME holders. This was exacerbated by the commonness of RMEs, which led to events designed exclusively for these players (like Provenance or Legion) that were consequently only done by a small fraction of the playerbase.
These factors are important for damage:
* Level - In non-Adoulin areas, the damage you do to monsters >99 is determined in large part by your level. +1 level gives you 0.05 Ratio and +4 Accuracy along with base stats, all of which can really add up. You cannot adjust your level (except by dying), and SE is moving away from using it as a determinant of damage.
* Haste - Important to have 25~26% in gear, but this is no longer difficult at all. +1% Haste is much more than +1% damage. We get the majority of our Haste from buffs.
* Accuracy - Important to have a 95% hit rate if it is possible, but unless you are fighting at a heterogeneous camp (like MMJSP used to be with Lurkers), the best way to compensate for this is to change the buffs that you receive. +1% Hit Rate is more than +1% damage. It's generally possible to cap your Accuracy with incredible buff/debuff levels, or if you can't then there's nothing you can do.
* D - Base weapon damage. This is one of the most basic determinants of your damage output, which scales almost linearly with it. +1% base damage is slightly less than +1% damage due to fSTR and WS mods. This can only be indirectly affected by buffs (Boost-STR).
* Attack - Important, but we don't get a substantial amount of it on gear. Red Curry Buns, for instance, give 150 Attack. That's about as much as all of your gear added together (at least pre-patch), and two minuets from a G-horn beat it. A "real" +1% Attack gives less than +1% damage over the reasonable range, and gives much less the more buffs you have. It's generally possible to max out your attack with incredible buff/debuff levels.
* Double Attack, Critical Hit Rate, Critical Hit Damage, Store TP, etc. - 1% of these give less than +1% damage in most/all situations. Some of these have caps of +100%, which are more and more hittable by the day. In general they grant diminishing returns and we can approach the top of their useful range with current buffs. Exceptions are basically Triple/Quadruple Attack, which are still not available in any substantial quantity and grant more than a 1% increase for every 1% stat, but still give diminishing returns.
So gradual buff creep over the years has left SE with scant few ways to increase player damage. They have said they aren't going to increase level further, and you can pretty much compensate for the evasion/defense of monsters you are fighting by bringing more buffers/debuffers as necessary. This leads to a very rigid party structure for events, which people hate. In order to release a new event that allows all players to adventure together on they jobs that they want, they needed to even out RME/non-RME players because content that's difficult for the first is impossible for the second. There is no factor in the above set that allows them to even out RMEs and non-RMEs so that both groups could participate in new events on jobs they don't hate without making 100mil+ weapons. Thus, the weapons had to go.
Now. That's what they did and I think that's why, but I don't understand why they chose to do it that way. They could have made Tier 1 weapons that were equal to RMEs (and thus wouldn't be discriminated against) instead of crushing them. This would effectively be the same result, with Delve boss weapons still being a lot better than RMEs, but I think it would have gotten much less community push back and there would be fewer problems in terms of DPS balance between different damage types. RME users would effectively skip the first tier of their content (farming weapons) and move on to trying to farm Delve bosses, but they still would have farmed Plasm to obtain Armor/Airlixirs.