This is quite unspecific as there are multiple tabs already; maybe you meant multiple windows? In any case you should ad details, like current problems and possible solutions.

we have started migrating the GUI to a web based format due to overwhelming customer demand. No marketing reasons were involved in that decision. The current hybrid nature of the GUI allows you to still go on with what you used in the last 15 years. NP, your choice. 
You could also make use of the CLI and REST API and build your own approach.

I never use the new home page/dash board. It's only the current fashion, driven by marketing "experts" not by customer's needs.

I like the  well structured GUI because I use it since 15 years. But I remember in the beginning it was very confusing to understand the changing meaning of the menue buttons on top depending on the changing content below. As well some configuration flows are not intuitiv from top to botton on some page i.g. device conficuration (SCSI adddress scanning/adding for physical or virtual tape libraires). 

Agree.

"Parts of the GUI (the part that you need every day!) has already changed to HTML"\

Monitor, IDB, Backup Specifications - these are really parts you need every day. Thay never changed cince HPUX GUI moved to WIN. 

The Home part is never needed by anybody I know (I'm administering more than 5 cells with thousands of clients).

"All of DP can be controlled by the REST API."

That's a lie. A lot of usual tasks cannot be done with a rest API. For example integration configuration or backup specification creation.