Do you agree with keeping cats indoor only?

I have two cats, they are indoor only. We live in a spacious two bedroom apartment currently but I have been wanting to save up for a house with a backyard so I can create a catio for them since I sometimes feel like they would be happy if they can be outside somehow. I know they are safer inside, but are they happy as indoor only? They have access to windows that we keep half open so they can get fresh air and look outside. When I am able to afford a house with a backyard, hopefully soon, I definitely would like a spacious catio for them to be able to enjoy the outdoors somehow.