To a first approximation, about zero people/companies have a problem they need solved which boils down to "I need a replacement for WordPress using a significantly more modern and better architected codebase."
By far the most common problem people are solving by using Wordpress is "I need to be able to fix website errors/typos without calling up a web development agency and waiting 2 weeks and paying $1000, and I need to be able update my homepage or add new pages which then fit into the menus and site navigation easily". Second most common problem they're solving is "I need a website which is easy and affordable to employ or contract experience people to edit and manage it for me".
Not a single WPEngine or wordpress.com customer has ever chosen or rejected them because they asked themselves "Is this run on a mess of legacy early 2000s vintage PHP and badly designed MySQL databases, or does the backend run beautifully architected and formatted Rust/Haskel/Go?"
> I’d posit I’ve been working with WP longer than you.
Perhaps? Maybe I'm older than I look... I've never specialised in WP, but my earliest WP work was on 2.5 back in 2008-ish after I jumped ship from a gruelling (and failing) startup. I was doing mostly WP sites at an agency during 2010-2013 almost all using cPanel shared hosting. In 2014 I moved to a place using Wordpress on Plesk shared hosting which had several really bad downtimes and performance problems, and searching my email the first WPEngine invoice came when I migrated a bunch of those sites to WPE in 2015. I've been happily using and recommending them ever since. Sure there are lots of people with more year's experience, but I've got a good 15 years or so, and I think that means my opinions are at least based in the real world.
WPEngine came into existence, at least from my point of view, because there was so much _bad_ Wordpress hosting available at the time.
My experience with 7 years of shared cPanel/Plex hosting on random ISPs or some generalised everything-to-everybody domain registry, domain registrar, web hosting company, SEO consultants, internet security snake oil salesmen like GoDaddy or Dreamhost, compared to specialist WP hosting with WPEngine - has been night and day. Where I work now, we happily refer clients looking for $5/month web hosting away to a few sole trader web devs we respect who handle those sorts of customers, and anyone who doesn't bat an eyelid at WPEngine's prices (plus our markup) gets hosted there with all the performance, uptime, and value add benefits they bring to the table.
I'm sure other people have different experiences. IU;m sure there are other dedicated WordPress hosting companies out there these days who are as good or better than WPEngine at least for some use cases. I'm sure there are other companies with as good or nearly as good offerings for less money than WPEngine. For me though, they are absolutely worth the extra cost for all the value add they bring alongside WP core and wordpress.org repos - except for people looking for lowest possible cost and are prepared to accept worse performance and less good hosting tools.
