Babies are not all the same!
I have not sleep trained any of my four.
Eldest: nightmare sleeper. Severe sleep apnoea, resulting in waking every 30 minutes or so. Operation to fix it not long after her 2nd birthday - amazing!!! I fed her to sleep until she was 2.5, then changed to singing/cuddling. Started to leave her to drop off just before she was 3 - did have a nightlight that slowly changed colour and gentle music playing. I never once left her to cry. She did cry for about 2 minutes the first night I didn't feed her to sleep, but thay was it. She used to wake up for a wee until she was 3.5, and we had a patch of nightmares when she was nearly 5, but other than that she's a great sleeper.
Twins: They had dummies. Amazing invention from my perspective! Kept them till their 4th birthday for bedtime. They did wake up and cry in the night for about 4 days after removing them, but were cuddled back to sleep. Only cried for nightmares/illness after that. (Do still have to wake the one with additional needs for a wee every flipping night though..... hope he eventually grows out of that!).
DD2: unicorn baby! Fed her to sleep, still doing so and she's coming up for 2. She wakes around 5am for another quick feed, and occasionally doesn't bother with that. She's just a very deep sleeper.
I've never sleep trained. I thought about it a few times (when the twins were a year old i thought about it a LOT) but never bothered. So no, I don't think most babies just cry forever until sleep trained. However, I think the age at which they stop varies widely. Children with additional needs it may be MUCH later than ideal. Sleep training may help it along.
If its what you need to do for your sanity/sleep, then there's lots of information out there. If you prefer not to, that's also OK - you wont harm them/prevent them from sleeping independently if you don't. You need to think about what you are most happy with and what works for your family circumstances.