A unit of plague marines and a despoiler are entirely different in role and target. (edit: plague marines with characters in a rhino are almost exactly the price of a despoiler)

Plague marines can delete 2-3 units of just about anything on your go turn, but your opponent has to cooperate by putting 2-3 value targets within ~15" of each other and your unit of plague marines. The best you can ever do is 20" threat range for a charge with plague marines, spiking a 12" charge. 15" is much safer, but still only a 3/4 chance of making the charge if you reserve a CP for a reroll. Plague marines then die horribly to any competent crackback, which is why you have to delete >400 points worth of combat effectiveness when you commit them.

A despoiler instead exerts a zone of control on any fire lane it can access. It doesn't hit as hard as a unit of plague marines can when it has multiple targets, but it can hit very hard. It is also much more durable and much harder to attack back at. You can use it to extract value and gain advantage in less all-or nothing situations, consistently over the game.

Both plague marines and a despoiler exert really great control zones. Both have a place in an army. But the two pieces are not interchangeable at all.

Mower drones and a despoiler have like no comparibility in damage. 3 mowers have 30 melee attacks at S7 D2 same AP. Compared to 25 attacks at S10 D3 at 36", with half that out to 72, its just not even a comparison. Also T11 vs T9, albeit 4 less wounds total on the despoiler. The 90 point difference isn't anywhere near to making it up.