Aside from being a long-standing obsession of Musk's, the thing about the 'everything app' is that it's the Hail Mary move which could make the Twitter thing anything but a dumpster fire immolating $20b+, Musk's reputation, and several years of his rapidly-shortening QALYs. If you force as many people as possible to subscribe, you can then flip them to the 'everything app' and bootstrap a big enough bloc of customers to matter that you control their demand. (cf. Stratechery).

It's not going to work, but it is the only story you can tell yourself and employees about how the Twitter saga ends in any way other than Musk losing interest and getting distracted by AI again and Twitter spiraling into the drain and possibly being dumped into bankruptcy by its debt load.