I have thought a bit more and I want to say this:
The purpose of magpies is, seemingly:
1) destroy raiders that get into friendly country
2) tilt small battles
3) gang up on vulnerable units like impalers/lances.
Magpies have a noticeable impact in early game, e.g. hitting a kodashi will halve its hp and send it running home, or they can tilt battles. However, magpies won't tilt all battles, i.e. vs swarmers they are a waste. Wouldn't it have been better to spend that metal on an extra kodachi of your own?
The answer then is, "well a plane can appear anywhere, anytime". A phoenix deals about as much damage of the magpie, with the bonus of killing swarmers. In fact, the range of targets for which to use a magpie against is limited. The argument then is, "a magpie can survive what a phoenix can't", and while true, the types of targets with AA escorting them are exactly those which magpie is useless against - i.e (shield)balls.
Very often, the temptation is to buy one more magpie. You just had a target you couldve killed with 1 more magpie. Blindly following this reasonable temptation - as it promises you the chance to kill impalers or lances, will lead to a rushed airfield. This is bad, as these resources could have been spent on 2 kodachis, and the builder someplace more urgent at this stage of the battle. Here is the weird situation where the unit best in the early game - magpie - gets as expensive as a phoenix. This is only the metal cost, though - remember that those 15s reloads cost a lot of energy - those 180 damage rockets are very cost ineffective. It would have been better to build 2 phoenixes and focus on econ instead of building that extra airfield to field more mags. Without an airfield you can have at most 2 magpies (each rotate in attack and reload), at which point you ask yourself "did I really go air just to micro 2 magpies?".
Debunking "surviveability" again: In the mid-late game, units get a lot more hp, and there are a lot more of them. Even a full magpie squadron (4 mags+1 airfield, 220*4 + 350 = 1230 cost) cannot deal enough chip-damage to justify their cost, the question then become "have the magpies killed enough to justify their cost?".
It seems then, that magpies were not made to kill raiders after all. We were all wrong. They exist for purpose 3). They are not for the early game, but for the early-mid game to punish unprotected lances (1000 metal, needs 3 magpies) and artilleries (700 metal, 2 mags), and exist exclusively for that role. But I want my magpies stronger! I demand MORE magpies!
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TL;DR Scaling up magpies causes logistics issue and is micro intensive. To alleviate the logistics issue, you build airfields which could have been spent on building better units or eco. There is nothing that can help you against the energy-hungry reload.
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What is the solution? Reducing reload is a good idea imo, it gives them more time for action and cheapens their energy usage, but airfields are still necessary unless the mags have a ludicriously low reload speed like 3 seconds.