I still do not understand how some players bought this. Arrival of new forces lowers the threat? So, if a hundred more maelstroms arrive I will be able to singlehandedly clear them all, right?
You're assuming that a Maelstrom is something akin to a carrier/shipyard - in which case, each would have its own independent supply/production of Thargoids, and adding more wouldn't make the others easier (though they each might get easier over time as they spread their own forces over more systems)
But what if it's more like a gateway to rapidly bring Thargoids from production facilities elsewhere into the bubble? Then there might be a single pool of Thargoids that they all have to share, and it's the total number of systems all Maelstroms are controlling which is significant.
Or it might be a command post or the Thargoid equivalent of a terraformer (which is why they're setting up around AWs), and the number of Maelstroms is basically entirely independent of the strength of the Thargoid fleet, but the cost of clearing a human-free zone around each one is diluting that fleet pretty rapidly at this stage.
Without knowing anything useful about what a Maelstrom actually is, I'm not sure I want to confidently say their behaviour makes no sense.