Essential is a theme built on Boostrap, Bootstrap wasn't around when Moodle2.0 came out but was added in during the 2.x series and as a result we have Essential and various other themes. Moodle3.0 is just around the corner, who knows what web design frameworks, best practice, or even user tastes and expectations may be by the time we get to Moodle3.5 let alone 4.0 - and that's before considering what Moodle itself will look like as a feature set in 5 years time and as Gareth has already pointed out, Essential will be quite dated by that time and if you are still using it, you may find your site needs a significant facelift to demonstrate a more modern feel by then.
As far as stability of any theme is concerned - well, there's the ones in core, but even they have changed since M2.0. Essential was originally created by Julian and Gareth took it on when julian stepped away from the community, and that's the beauty of open source - if, for any reason, Gareth decided not to continue supporting Essential in the future, the code is there and someone else (or you/your institution) could continue to maintain and update it (or your own fork of it), i continued using Julian's version of Essential after Gareth had taken it over for quite some time, simply because I was happy to maintain the customisations I had put into it until such a time as I felt we needed to refresh the theme (at which point I selected what i thought to be the best theme to move to - Gareth's version of Essential ). Even Partners move on and things change.
in most cases most themes will be stable across several versions, particularly the big ones like Essential or BCU as the wide user base encourages that stability. But by the time Moodle4.0 is around there may be better and better themes developed which embrace web functionality we haven't even considered right now (browser based video chat as a Moodle component anyone - a la Firefox Hello?). HTML6 may be around with CSS4.
Also bear in mind that changing theme doesn't necessarily have to result in a huge change in look and feel, just don't use some of the new additional features and apply customcss to the style.