I had an old Elna SU62 back in 1978 till 2000 and I never took it in for servicing ever. Heck I can't remember if I ever oiled it and I'm sure the poor thing was supposed to be oiled but she was such a trooper, never complained. Wish I never sold her now.
As for the newer machines, yes they have sealed areas so no oiling by us. Had my machines in storage for 4 years so when I got settled I took them both in for servicing. Its been a couple years now since then and I just cleaned out the bobbin area real good after every bobbin change. Got my super duper mini vac attachments out the other day and really gave them all a good cleaning. Have 2 sergers that have never been in for servicing. They get so dirty with all that fabric and thread flying about so cleaned as good as I could. Just found out my Babylock Evolve has a little hatch at the bottom you can remove to get inside deeper, Never noticed that before. Maybe I should pay closer attention to my babies than I do.