I've been doing both, or rather different kinds of combinations from almost full PHP to PHP with jQuery/Backbone/Angular, and since a while React SPA to Node based including Nextjs. Also just using dependency less JS and htmx as well. All depending on the scope and requirements of the particular projects. For side projects I've built stuff with quite few other languages. This is for professional context.
My conclusion so far is that most of the criticism towards JS based solutions is largely outdated if you pick your tools and libs well. The leverage of something like Nextjs and similar is pretty significant over traditional SSR, even for cases where the latter made more sense a while ago, increasingly so. I don't think PHP is going anywhere in the near future, but I see fewer and fewer reasons to use it at all.
1. When people talk about isomorphic code for frontend related stuff, they often pick form validation as the example. But this is just one of many things. It's also the tooling, testing, types and other integrations that you miss if your frontend logic is spread across language boundaries.
2. Unoptimized performance of a PHP vs Node/Next application is quite significant as PHP needs to recreate the whole application state with every request.
3. Frontend without or "minimal" JS is a pure luxury that you almost never get to have and if you do, it comes with its own complexities.
4. Websockets and other features are a pain to use (if you can use them at all) in PHP.
5. Development is faster and more responsive with a React/Nextjs/etc. based implementation. The feedback loops are faster, tooling and libraries are more integrated.
6. There are quite few good libraries popping up in recent years for JS that give you more leverage than what I'm used to from PHP.
7. Commoditized hosting PHP has been one of its strengths, but even that is being outcompeted slowly but steadily.
8. By default there are things you cannot do or only with additional (unnecessary) effort if you split your frontend logic into two places, rather than having a single, comprehensive codebase. Frameworks and libraries like Next, Remix, this one and others are leveraging that. Yes it gets more complex but you also get more optimizations
Please note that I don't particularly like either JS or PHP. I think both languages and ecosystems are quite terrible in their own ways and have to be tamed by pragmatic developers. So no emotional attachment there.