Car Seat Quandry

I still use both Calebs travel system carry car seats occasionally. He has a maxi cosi fixed seat in the car too but I find the carry car seats so handy sometimes.
Hes 17 months and 22lbs but he loves snuggling down in them for a sleep. This concorde travel system is especially handy and very cosy to sleep in. Today we popped to my mums and as he was sleepy when I left the house I put him in this seat in the car and put the pram in the boot. As expected he fell asleep in the car on the way home, so when we arrived I attached the seat to the pram and put him in the back garden for an afternoon nap. He was cosy and happy in there.

I sometimes wonder if theres a reason why the advise on the seats is up to 15 months as this seats weight maximum is 27lb which he is no where near. I wonder if these guidelines are rough or whether theres a reason they attach them to these seats??? Im starting to worry how much longer he'll be able to use them.

We also have a maxi cosi mura travel system and sometimes i take him shopping in it so he can sleep on the journey and just transfer to the wheels for the shopping.

Does anyone else still do this at this age or am I mad??