Have to say I just love developing and demoing my themes on multisite installs, with one set of shared plugins.

I make a change to my core plugin and each dev site / theme that use it gets those changes right away. Configuring certain plugins at the network level is very convenient. Multisite is not for every use case but there certainly are many situation in which it provides convenience and even cost-savings.

Is it perfect? No. WordPress ought to have been modernized with a total rewrite before Gutenberg, with a better network mode that can scale without nightmare-level effort. But what we have now is definitely better than nothing. The idea to deprecate it is surprising.