SharePoint Site Collection in SPOnline

I think it really depends.  In general, I would think the strategic direction would be not have many sub sites and to have them be Site Collections.  You can then leverage Hub sites to associated common sites (the old sub sites).  This will allow for a lot more flexibility as the business structure may change and re organizations happen, etc.  

Do these sub sites all inherit the same permissions as the (top level Site Collection) parent?

Do you roll out content from sub sites to parent (top level Site Collection)?

What drove the decision to make them all sub sites in the beginning?  

There may be a valid reason to still maintain sub sites, but I think in general MS is making the recommendation to only do sub sites as the exception and to go forward with Site Collections and hub sites.  

For the record, I am old SharePoint dude, and it toke me a while to get my head around this new approach, but I am embracing it now and I am seeing the real long term benefit to there new approach. 

This will allow me to spend more of time focusing on the more complex business process and solutions using SharePoint and other O365 tools, and not spending the majority of my time changing and moving content.