line comes very close to convincing me as well. But I think
we really need to see relativistic motions or phenomena originating
close to the Schwardschild radius to clinch the matter.
So let me summarise. We have evidence that probably most, if not all, galaxies with masses of order the MW mass or higher, have a SMBH in their centres. It also transpires that the mass of the SMBH seems to correlate with the mass of the galaxy: the more massive the galaxy, the more massive the SMBH. What does that tell us about the formation of the galaxy or of the SMBH? We don't know (yet!).
Also, if most galaxies contain a SMBH, then why are there so many times fewer QSOs than galaxies? Indeed, QSOs are rare, so only a very small fraction of SMBH is active and produces AGN activity at any one time. The best guess is that there is no gas accreting onto those SMBHs: these inactive QSOs are starved for food. Still it may be a bit surprising that so many black holes are so very black - some gas is lost by the stars close to the centre, which somehow must be accreted onto the black hole without producing much radiation.
And finally: if a star ventures too close to a SMBH, then it may be torn apart as a result of the tidal forces from the SMBH. This could lead to a very bright flare, which would last for of order months. The realisation that SMBHs may be quite common provides fresh motivation to search for such flares. Upcoming missions designed to detect SNe will find these flares - if they exist.