Dear Meindert,
To your questions...
1) Unfortunately, I do not have effort estimates for replacing BW-BPS by BW-IP within the campaign planning scenario.
2) From my experience, up until now most customers leverage the standard integration with BW-BPS, since campaign planning is key to enabling the end-to-end process for budgeting in CRM. Even though BPS is old, it will be supported (in maintenance mode) for at least the life of NW 7.3
(currently Dec. 2020). I agree there are rising concerns on this topic and SAP is starting to look into options, but there is no firm roadmap to announce at this time.
3) I cannot confirm the feasibility for campaigns, but in the TPM area, they are leveraging BW-IP for embedded promotion planning.
Regards,
Anik