Not really sure why you say that whilst TL3 tries to solve exactly that issue in a simpler, classical way [0]
I love the idea of being able to use the usual Rails helpers combined with JS in a React-kind of way and at first sight that's what TL3 could manage to do.
[0] by that I'm referring to the clean HTML that still looks semantic and has all the normal separation of views and styles whereas in React et al if one's inspects the code is just looks horrid.
Regarding SPAs the isomorphic approach seems really contrived at this point and for most web apps TL3 seems like an elegant solution whilst keeping the full power of Rails intact.
Sure there are cases with heavy UI apps that require components et al but for my use cases TL3 + Rails5 looks like a breath of fresh air, maybe a middle step to full SPA.