Rasmus my dear friend. I see the move to Microsoft has clouded your objectivity :-) Unfortunately, all the statements are fully correct.

Claim: "Microsoft is no longer investing in Skype for Business".
The online service description for SfB clearly states that as of Oct 2018 Teams is the primary client. At enterprise connect, the communication was - users of SfB online will need to migrate to teams, no further development on the cloud platform. SfB 2019 server is the way to go if you want to continue using SfB in the long term. Extended support contract pricing was released and accompanying EoL notice. Essentially no further development on Skype for business. If you look at the official UserVoice forums for SfB, there is no BU feedback on any posts.

Claim: "Multiple tenants are required for external communications"
100%. The super construct of Identity in MS Teams is a tenant. External communication is defined as communication with a user or channel on a tenant that is not your own. So a minimum of two tenants is required for external collaboration. This introduces the concept of guest access. Webex Teams doesn't have "guest access" because it carries the notion that the external user can do less than the user who owns the space/channel.

Claim: "With Microsoft, you need to invest in an E5 license for messaging, meeting and calling"

Whilst you are entirely correct that technically you can purchase phone system as an addon, it is priced such that it often doesn't make commercial sense to purchase a single add on capability when you can acquire 6x capability for 2x the price. So the addon is often given away for free, for the initial term.

If you want to discuss, you have my number :-) Always great to hear from you.