Linear, linear, linear

[Shurrikhan]

Blizzard also allows the sale of carries and power leveling in their Trade chat at Gold levels that can only be achieved via Gold Buying or WoW Tokens.

Again, no, advertising carries is a 3-strike offense (silence -> suspension -> ban) even if for gold/tokens. (While doing so for actual money has always been immediately bannable.) This was made official / no-tolerance for Group Finder back in July 2018 and also in Trade/General/Shout in Jan 2022. Meanwhile, our own Party Finder...

Blizzard has even announced that they are going to allow trading between servers within the same Realm which only serves to benefit Goldsellers and market manipulators. Many servers are about to have their economies destroyed by this change coming in the next patch.

Unless there are currently servers who are already themselves ruined economically or uniquely beset by market manipulators, no, the change will not suddenly destroy servers' economies. They'll simply become a single economy, rather than some categories of goods being faintly cheaper or more expensive (larger gaps the smaller the server, due to the ease of market manipulation) on this or that server compared to their realm average.

Note that market manipulation is typically least friendly to the average player (the most oligarchical, so to speak), on the smaller servers. Outside of market manipulators, players primarily stand to gain from enlarging the market size.
And no, I can't really manage sympathy for those suddenly unable to constantly buy out whole sections of small servers markets to keep prices high in the interest of their profits over the average players' access to good per at least halfway reasonable prices.