Why did 15 year old dog die?

Hello, My dog was 15 years old and she just recently died last thursday. My mother is really upset because she feels guilty that we didnt take her to the vet sooner. She started throwing up on sunday and it was dark brown colour. The next day she harldy ate and at night she threw up again. For the next couple days she was throwing up brownish redish which we were thinking it was blood. We made an apt on wed for thursday but on thursday morning she passed away. She was breathing really heavy and hardly ate anything, her eyes and nose stared haveing green snot coming out of them before she died and was not able to walk near the end. In 2004 she started having leg problems and so we took her to a specalist. In 2007, her back end gave out and she could no longer use her back legs. We took her to the vet and talked to some people and we ended up putting her on steriods which worked and she was able to walk and she was on them for 3 years. She had lots of lumps on her body which could have been cancer or something else the vet told us. I know this may be confusing but do you think this was just apart of aging? If we took her to the vet do you think there would be something that could have been considering her age and past? Please let me know. Thank you.