I'm pretty sure the intention is, as you say, to allow players to try out the "complete" game, and maybe encouraging people to dip in and out when new updates/DLCs are released, as opposed to getting people to subscribe to it permanently in lieu of actually purchasing the game and all its DLC.As a trial, it's a really appropriate way to bypass the DLC mountains. However, it's still expensive for just one game and this makes it a stupid long-term spending. Though, I guess it's in line with the already overpriced DLCs.
You might get a month's subscription to decide whether you want to actually fork out the cash to buy it when a sale comes around, or you might be an intermittent player who only really comes back to try out new content and doesn't play much, or even at all, in between updates. In that regards it allows people to enjoy the game and all of its content without having to make a major investmen, which seems pretty good actually. It's still not really of any interest to me, but I don't doubt there are plenty of potential customers for this out there.
There probably will be some who do subscribe long term, but I don't think Paradox is responsible for those who decide to make obviously poor decisions in terms of immediate versus long-term costs.