As someone who has been doing frontend, backend, systems administration and general black magic since before the jquery days, I can tell you that managing frontend requirements was absolute hell back then. You literally had to put the script and style tags in the right order in your header or footer and just kind of hope the timing worked out. People how work exclusively on the backend tend to underestimate the complexity of frontend builds in the same way that non-technical people underestimate the complexity of all software engineering. The truth is, something as common as an autocomplete input is an application unto itself, with multiple dependencies all with load priorities. I don't have to imagine what this was like back in the jquery era, because I experienced it and it sucked.

I noted this earlier in the thread, but the fact is that any user-facing application with a UI is going to include JS/TS and CSS, and therefore is going to include a frontend build of some kind. Any build included in an application comes with the requisite build orchestration for all environments, so Docker config, build specific config, CI config, etc. If you add a second language to an application, you add a second build, and the cost of setup and maintenance of your application basically doubles. Not only that, you now have to have engineers proficient in multiple languages on your build. Most engineers are really haughty about the language they write and the part of the codebase they prefer to work on, so finding someone who is equally comfortable AND proficient writing python applications and frontend code is extremely hard. Most likely you'll get a Python engineer who will reluctantly write some awful React and complain about the entire time, or vice versa.

So if you come to me and try to add some Python or Ruby or god help you PHP to my single build Node/React application because you think its somehow going to make things simpler, I will simply eject you into the sun.

I'll write an API in any language that makes sense for the performance requirements. But if the app has a UI, absent any legacy requirements that force you to do otherwise, it 100% absolutely has to be entirely in JS or TS or you are shooting yourself in the foot.