Couple of notes:
First, it’s not uncommon for you to reduce the power of old trinkets. Also, Myth track Nerub’ar gear was 639. That is the level of veteran gear in Liberation of Undermine. With a 39 ilvl jump each season, and targeted buffs and nerfs, it’s not unreasonable to think that champion pieces next season at 13 ilvl higher would be better than myth level pieces.
Myth level pieces at max level also assumes someone is running content to get gilded crests. If someone is doing that, they probably are going to be able to get at least hero pieces next season. This is actually something that wasn’t an issue in Dragonflight, because pieces started all the way at Veteran 1 and needed crests spent on them. If Dinar pieces started at hero one and went up to Myth 8, it wouldn’t overly benefit players with myth pieces who don’t already run content to get those pieces.
Secondly: Dinars were available in DF S4 on the second week of the season, and went up to more than a half-dozen items. This is three pieces (originally going to be two), not available until more than halfway through the season.
Thirdly: there can be multiple unlock criteria. Players who clear high-end seasonal achievements like Let Me Solo Him, Keystone Master/Keystone Hero, and Ahead of the Curve seem like a reasonable unlock criteria for mythic pieces.
Fourth: Raiding is one of 3 pve pillars. It is the pillar with the most access to high-level loot. Delves don’t get myth pieces at all, and mythic plus has only 3 items each week to choose one from. If it is intended as bad luck protection, why can players who run M+ not select a piece using dinars, at myth level, but raiders can? Why can’t delve players select a hero item?
Lastly, if mythic pieces are such a concern, why make them purchasable at all? Just lock it at heroic for everyone and let players do as they wish with that, like you did with heroic-level pieces in the Forbidden Reach.
While I appreciate that there is a response, this is a lacking answer. Either commit to them as a bad luck protection to be able to target max-level gear from each content pillar, commit to them being dinars able to purchase raid gear, or commit to letting people snag a few cheap hero pieces.