Removing the aggro cap wouldn't make Tankers and Blasters overly-appealing. If anything, nobody but a roleplayer or someone wanting to tank on the cheap should be choosing a Tanker over a Brute now, with the aggro cap.
However, the aggro cap is where it is because the servers can't handle that many active AI/mobs. It might be something that could be optimized further in the code, but I kinda doubt it. All games have struggled trying to manage a lot of AI at once. They mostly fake it by grouping and instancing, dividing up processing cycles between different subgroups, and just turning them off when they're not onscreen or outside a given radius from the player. The CoX server probably already does all these things, and while every enemy is its own little state machine, it's not like we'd be happy if they were made even stupider.
An actual programmer may want to correct me here, but I'm thinking the only way to bring back those hordes as the norm would be through beefier hardware, relative to the player population. New code might be written to take advantage of specialized hardware (some kinds of AI use GPUs for large-scale parallel processing tasks), or make better use of current processor architecture. Ultimately, though, I believe it's a matter of raw processing power.