Point is that all apps tend to get bigger. So why postpone necessary?
I think that I implicit answer this questions in my 2 long answers after it. But for the sake of clarity, I will answer explicity.
I don't think that all apps tend to get bigger. I make for many customers web apps and mobile apps that simple remain as-is since first release, for years. Or they don't have cost for further development, or they close business for other reasons. Or they are happy with current stage. I have even my commercial social network application still running with PHP 5.x and PhalconPHP 1, still being used by several customers. Now nearly 2 years not updated apart minor fixes. Technology move fast, and I see that most of your application is more easy to refactor than upgrade to new one. This happen with an Angular 1 app, with several Ionic app, Phalcon PHP, and I think also for switching from Laravel 4 to 5.
I don't start a project thinking on abstractions immediately, I do that when in phase of polishing the code, when I see necessity of extraction of code. Refactoring is part of that process.
I think an inexperienced programmer starts thinking about abstractions at first, I used to do that too. But it is impossible to predict the evolution of the code unless you do a month of analysis with UML digrams and everything else. They raise development costs that customers are not happy to pay. I'm not saying they don't need to be done, on the contrary, in some projects they are essential. But in small to medium projects they are not. I prefer to keep the code as simple as possible at the beginning, even if that means repeating the code a few times. And only towards the end if I see the need I extract the code. But often this is not necessary.
Again, if you are large company working with many developers, then you may think differently. But being a solo developer or small team, you can't afford to work as a company.
Now, Jetstream/Fortify offer abstractions for handle 2 frontend, which I will never use. And this abstraction make code more complicated to manage. Something that I and other developers don't need make your starting point already complicated.
I hope that I did explain well myself.
May Jetstream be with you.