[2022] If this was ur plan why release glavier and destroyer?

This was addressed directly in the roadmap: Team Update & June - July Roadmap - News | Lost Ark - Free to Play MMO Action RPG

Our teams work with Smilegate RPG to evaluate a variety of factors when looking at which Advanced Class to release at a specific time, such as planned balance updates/reworks, relevance of the abilities and playstyle to other content, community feedback, and more. The release of the Glaivier and Destroyer Advanced Classes provide a good example. Our teams knew we wanted the Destroyer to release at (or around) the Valtan Legion Raid for players to begin leveraging the massive stagger damage in end-game activities. However, releasing the Destroyer shortly before a planned rework wouldn’t be a great experience, so we pulled forward the fan-favorite Glaivier Advanced Class into its place, and released the Destroyer alongside Valtan.

The next class to be released is indeed a Mage class, like you called out, because there are currently only 2 mage subclasses available.

Glaivier was specifically pulled up because of its popularity as a fan favorite class, which came directly from community feedback. Again, Destroyer was chosen because of the stagger skills and their usefulness with Valtan, which is “relevance of abilities and playstyle to other content,” as well as the new rework. Both of these things were stated directly in the paragraph you replied to.

The “drip feeding” class release cadence that folks are upset with =/= the order classes are released in and how they are chosen; these are separate topics