Custom theme, life expectancy

I built a custom theme back in… probably 2010-ish. It’s using calls like
get_header(); get_footer();
and

    <?php while (have_posts()) : the_post(); ?>
        <div <?php post_class() ?> id="post-<?php the_ID(); ?>">

The only custom function in function.php is a register_sidebar call.

My question is: is it possible to predict for how much longer this theme will continue to work? Are the examples given still WP core snippets, and for how long will they continue to be so? (I no longer work in web design, and have become lazy – I only ever use a framework like Divi when I’m occasionally building sites nowadays…)

And yes, the client should have their site reworked –?for responsiveness, for one! – but lack of funding has put this off.

Thank you in advance for your advice!