Both the Apple Watch and Pixel Watch come with cellular versions that support driving directions and phone calls and texting and on the surface provide many of the functions of a smartphone.
If you're interested in this idea and drive a car, I would encourage you to experiment by leaving your phone in your car's glove compartment.
Directions, payments, communications, calendars, and notes are the essential things, and especially with Keep single note tiles watches do most everything pretty well.
If I had a cell enabled watch I might consider trying, but I don't want to pay for the extra line, I just wanted to be able to put my phone in a backpack most of the time.
Using them takes active effort though, it's not like you automatically look at your watch for the time or to check a notification, "Check phone" is probably stronger than any other reflex.
Everything else requires at least a tiny bit of thought but "I heard a noise gotta check it" is effortless and you have to actively learn to not look at it all the time or physically leave the phone far away if that's your goal.
I view smart watches as a convenience, but smart phones are a necessity.
I could do that for special occasions like sports, short hiking's or going out to have fun but not much more. Oh wait, I wouldn't be able to buy bus tickets, show purchased tickets at the cinema or show an ID (soon to be implemented in Sweden) so skip the last one and we ended up with an expensive sports watch.