Well, not a false economy if you spend the same/less on a rotating birth to 18kg seat as you would have done on an infant seat and base. I defo think it's worth exploring.
The PP talking about an isofix base - for many infant seats, you have a base which is semi-permanently fixed into the car using either isofix (metal hooks built into the car, the car seat or base has corresponding clips, which just click onto the hooks for easy installation) or using the car's seatbelt. You then just press a button on the car seat, lift it out of the car and be on your way. If you use a carry car seat without a base, you have to strap the seatbelt around the seat every time you put it into the car. It only takes a few seconds, but that can be annoying if it's raining or the baby is crying or you have a toddler to sort out as well. Usually you have to buy the base separately and often they cost £100+ on top of the seat itself.
Do you know why the Cosatto seat doesn't fit your car? As this reason might rule out other seats as well. One issue with a lot of the rotating ones is that if you have very sloped back seats in your car, the baby ends up tilted much too far forwards and that isn't safe for them. With a seatbelt fitted infant carrier, or some bases which add a tilt or recline function, this kind of angle can be abated. There's one rotate one which solves this too but it's very new and very expensive.
Or if you have an older car, you might not have isofix hooks, which would rule out a lot of seats as well.
I would add that the Cosatto infant seats which come with prams are OK but their larger ones for toddlers, rotating ones and all-in-one up to age 12 ones don't score very well for safety. The best rotating seats for newborns IME are GB Vaya, Maxi Cosi Mica, Joie i-Spin 360 and the Besafe one which avoids tilt issues but is very expensive.
There is also the new Axkid One+ which will take you right through until age 7 but that's very expensive as well! It's another completely new type of seat which is why the large price tag.
If your Cosatto pram has the option to buy car seat adapters for "multiple brands" or mentions Maxi Cosi you can use a huge range of infant car seat brands on it. So no need to necessarily stick with the Cosatto infant seat if you decide you want a carry function.