Modern subsites?

i would say that from a performance viewpoint, resolving sub-sites has always been an overhead as it reduces the amount of work that can be delegated to SQL by the need to translate internal sharepoint only meta-structures. Now that the old limitations around site collections and database sizes have gone, it makes sense to evolve the granularity and use site collections as the atomic site so that performance can be accelerated. it is only because we have got in the habit of subsites that we are nervous about moving away from them. The advantages they gave in inheritance are much outweighed by the problems of broken inheritance. The inability to separate them in SQL can become a problem as a site grows and then forces a restructuring.

For small segregations then List/ Library structures can be used instead of subsites to partition data physically for security purposes.

The https://www.unily.com/insights/blogs/microsoft-ignite-2017-announcements-you-need-to-knowarchitecture looks like a more formalised evolution of cross-site publishing which delivers a very robust and flexible system where I have used it before. excellent pattern that really leverages search.

perhaps those with a love for subsites could bullet point their advantages to see if we can suggest how that functionality would be designed into a site collection approach.