Aside from the safety concerns with children spending too much time in their car seat, most children don’t like being strapped into their car seat.
Some babies don’t tolerate the car seat, making even short trips miserable. In this case, the Doona (or any travel system for that matter) is not the best purchase. For babies who are intolerant of their car seat, transferring them to a traditional stroller is likely a better option. For babies who can’t yet sit on their own, you will need a stroller that lies completely flat (or has a bassinet).
Even babies who tolerate the car seat well still may prefer to sit in traditional strollers. This is especially true as they begin sitting up un-assisted (usually around 6-9 months). Most babies would prefer being able to sit up and take in their surroundings as opposed to being strapped in an infant car seat. Around this age, many parents also choose to forward-face their stroller if it was once parent-facing based on the cues of their child. The Doona doesn’t have the option of forward-facing (only parent-facing). Allowing children to sit in a traditional stroller allows freedom of movement that being strapped in a car seat just doesn’t allow.