First thing, you have to acknowledge that planned obsolescence was a much bigger part of game design in 1999. Secondly, the Vasudan fighters are much more usable in FS2 than they were in FS1, as such you also have to include them an on progression curve.
If you acknowledge these points you can see that Myrmidon is limited by outside factors than internal balance.
But it has its good points, for one with 3 secondary banks it is one of the more versatile fighters of the Terrans, hindered only by the small capacity per bank and lacking access to the Harpoon.
It's rotation time could also use a bit of the buff to put it between the Perseus and the Herc II, instead in the 4s.
Now there is also more I would like to tweak, but that needs to be embedded into overhaul of weapons and damage that cannot be limited to just the Myrmidon, so I will not elaborate on it more.
It's not a successor of Apollo. Perseus is.
The Perseus is a successor the Vakyrie in more than fluff.
It did loose the top speed and afterbruner speed, but has better acceleration and rotation times. But more importantly it is far more reliable, especially in the newly important bomb intercept role; its improved shields and subsystem health (the percentages are the same, but with higher base HP and smaller radi) make it more durable against the fallout from shooting down bombs. Adding to that both primary banks can now be used against any target (Valk's wing guns are only really good against larger targets) and the second secondary bank, which gives it more mission variety, it easy to see it as a holistic evolution, even if one performance metric, namely speed, has gone down.