i wouldnt be too worried about the cars being phased out, they will still be around for more than the next 40 years (your working life) although i love cars i didnt go into that industry because the money isnt so good, its pretty dirty work (not a real big issue) and theres so many of them around, which makes it harder to find work, and will keep the pay rates low IMO.
i personally am doing my boilermaker apprenticeship atm, because i dont mind fabrication and its something i wanted to learn, if i was to do anything else, probly plumbing (good conditions, pay etc).
the other thing is, all my mates that have become mechanics, they've all lost the passion and cbf workin on they're own cars, and as a guy that i work with told me 'working on cars is my hobby, i'll do it when/how i want to' and it makes sense to me. obviously you wont be pro right away, but i've found theres always plenty of people out there to help/show you things, and i've learned alot of things this way
that said your starting tomorow so do it and see what you think of it, if you like it, stay with it. just find something you enjoy and the money will come