gmail.com may be around for a long time. But you may decide at some point, for whatever reason, that you do not want Google Gmail to be handling your email. It's pure hell to change to a new email address, much harder than changing a phone number, because senders' records of what they *think* is your email address last essentially forever. Not at all like a little black phone book.
Domain names are cheap, as low as $5 a year depending on the Registrar you use, but certainly easy to find for less than $10/year. Buy one for your personal use. They're not just for business any more!
Set up your private/personal/vanity domain so that email gets handled by whatever email service provider you are using for the moment. Change providers when you want, for whatever reason. Your email address doesn't need to change.
Benefit: if you own the whole domain, you can change the "local" part (to the left of the '@') any time you want if the S/N ratio goes bad due to spam.
It took more than a century to get phone number portability, so that you can take your phone number with you when you change phone service providers. It's built into the email system, if you use it correctly.
And since domain names are quite cheap, compared to the "Old Days", by all means use a different one for a hobby or small business, from Day One, separate from your personal domain. Then you don't have to struggle to separate your personal life from your hobby-become-business from your small-business-just-sold.