20-Year Prospect
Popular nail styles will continue to change and evolve, so it'll be important for you to stay on top of the latest trends and provide the gels, silks, topcoats, and hues to match. Manicurists' tools and tech aren't likely to change much (or be handed over to robots), so your job is likely to be more or less the same for the next twenty years.
The only competition you may face is from DIY YouTube channels that teach people to style their own nails. These have been growing in popularity and make it a lot easier for people to learn new techniques at home.
If they have the skill to pull it off, that is.
Other people who are too lazy or busy will still be coming to you. Getting manicures is a social and relaxing experience for most—they're paying not only for the nails, but also the service and delightful conversation you provide. The styles, shapes, and colors will certainly change with the times, but you'll be there to make sure they look great.