No one knows what's going to happen in a few years, it might be there, it might not be. Even larger players can disappear, in the early days there was Lycos, they had 'free' email, lasted for many years, then they collapsed.
If you want guaranteed longevity, you need to pay for it.
If you want a better chance of longevity, they need to have it fund itself (which means ad's or selling your data).
If you want a small chance at longevity, then expect something for nothing.
Running an email service costs money... that money comes from somewhere, they don't do it from the goodness of their hearts.