We know you wouldn't. But it doesn't matter. It is a given that if you want to sign a top FA you'll eat some years. If the Phillies could get him for just three they'd jump at the chance and happily not watch him pitch a single game in 2020 if that's how it went down. But he'll probably get four or five anyway, I don't see Boras settling. So I'd take him at three but otherwise I agree it's probably best to stay away from him given the question marks combined with the fact that he also costs you compensation.
If the Phillies did pursue him it would be based on their familiarity with him, which can be either good or bad, and on the hope (or analytics-driven belief) that they are more likely to see the 2015 Arrieta again than to get that kind of performance from Cobb or Lynn. If they don't believe that, they won't try to sign him.
But it's also not a choice between Eickhoff and a FA, any more than it is a choice between Nola and FA. They are all going to pitch, and in that remaining group of less-proven guys, someone is going to get injured and someone is going to flop (and Eickhoff and Nola remain injury risks as well, but all pitchers are).
Of course if you want Harper overpaying to get Arrieta would be a good way to follow in the Nats' footsteps with Boras. But we know Harper is probably not the Phillies' first choice (nor they his).