It's not a monopoly if it is hard to migrate from. SAP doesn't go out of its way to prevent you from migrating out of the HANA database. It's just that by the time you need a software like SAP, whatever you're buying becomes business critical and hard to migrate from regardless. And SAP is good enough to handle all kinds of edge case scenarios reasonably strongly. SAP doesn't even engage in monopolistic practices like what Microsoft or Apple do.

SAP and Salesforce are pretty much in the same bucket imo. The reason there is no Hubspot for SAP is that most smaller companies don't really need an ERP system.