The fact you ask this question gets at my point I think.

For several years I've been writing a large computer algebra system(CAS) that runs on a webpage. Every time the user puts some input into a text box the CAS runs. Depending on the input it may run as many as ~40k lines of code. There are no coherent lines upon which to split the CAS as far as anyone developing it can tell.

The CAS must run on the browser both to deliver on real time performance requirements and to keep server costs manageable (certain inputs will get even high end CPUs humming).

If breaking this SPA up is possible, it's not apparent even to engineers with >10 years of experience developing highly complex applications.

Other similarly complex applications run on the web, even if it's unusual.