Hi GS
One of the key things is simplification as others have already pointed out. Now in terms of dollar or sunsetting the systems, specifically BW, yes, you have embedded BW components in S/4 HANA which you can use. However, that depends on what kind of BW Reports you have today that depend on MSEG and MKPF. Additionally, in addition to logistics, you may have other reports and calculations in BW, which you probably may not have in S/4 HANA as of today and it may not be possible to sunset the BW completely.
You will have to analyze the same how the embedded cubes for Finance and logistics fulfill your existing reporting capabilities in BW and check if S/4 can completely meet them or not and than take a call. That will also help you come or check $ values for investment in S/4. Also think of the fact, that with S/4, you probably will need just one cube rather than multiple cubes for different tables, so it will also reduce your load runs for BW if you choose to go with S/4. This should in the immediate run, bring down your time to period close for that matter. So you can find $ saving there. Some of the reports or queries can come to HANA Live views which are in fact much better than BW in terms of usage and these will be real time.
So essentially, you will have to look at your existing reports and see how much of that can be translated into $ values should you choose S/4
Regards
Sanil Bhandari