Here are my 2(000) cents about these things.
First of all i’d say it all depends from that the final product. It all comes down to the CONTENT and GOAL in my opinion. Technology is just one of the tools.
Will it be a presentational website that has just some contact info and text and pictures? Do it PHP, adding react/angular to it can be overkill unless you really want to give the visitor a bit of a different experience with transitions etc. Still you can do pretty nice and amazing things with some minor js/css animations so no need to do it overkill. Also check this post by USTWO they have a wp-backend/react front end. Just look at the architecture of the site and tell me what the difference is between a PHP one or the visitor experience? Is it easier or better to read then a php with some ajax or css animation? Is it faster? (try pingdom or gtmetrix with their site).
Doing things in react just because everyone else does it or because it’s hype is pointless and as you said maintenance and learning new things is time consuming.
For example i tried out some example themes with react and imagine this, they are missing simple things like the “read more…” functionality etc. so you’d need to recode those functions probably from scratch just so you can use them on your react blog and there are tons of other such elements.
BUT… if your focus will be building proper Apps with functions/experience non existent or totally different to what WP has now (so not redoing EVERYTHING wp already does like blogs etc.) then you could probably find better ways/frameworks of doing it (meteor+react maybe?) then WP.
If it can be done it doesn’t mean it must be done. End of the day what you put into the website, so the branding part, design, look & feel, the written content and the main message (which is a goal of each website) or a special service that makes things easier (which is one of the goals of apps), all these things should be your starting point and not the technology part.
If you’ve started a project and ask yourself will we do it in react.js/WP, you’ve started it from the wrong point. Read the HumanMade side of the Ustwo project from above and you can see that they were brought in in the middle of the project after design and everything else was already done.
Also Ustwo isn’t a great example in some ways as their goal was to test/experiment/learn and create something they can show off to others with their website. It isn’t a perfect example of how things should work it’s just another experiment (but a good one) of how things could work in the future.
I think it’s too early to tell which way things will evolve with Wordpress & JS so until then do what you do best.
Your clients/audience won’t care if it’s JS/PHP etc. they want things to work like they should and have VALUE for their company or their audience. Focus on making that better.
Also note that i may be wrong just in case you shift your companies focus because of this post