I made SBS my "bread and butter" back in the day. I've been in this game since NT 3.5 and 4.0, I remember "Microsoft Back Office Server"...the great grand daddy of SBS, and I REALLY got into SBS when SBS2000 came out. Devoured books on it, met Harry Brelsford a few times (Harry was a big promoter of Small Business Servers, writing quite a few books on it, and hosting road shows for Microsoft).
We made big money on SBS, in addition to the servers ( back then, a good 5 to 8 grand on hardware), however the big thing was consulting, we turned on the features and taught clients how to use all those features!
As SBS faded, and Essentials came/went, and 365 came out, we dreaded this big loss of income. Dreaded going from nice thick steaks for dinner every night, to....eating ramen noodles every night. Initially with Microsofts first program for partners to resell 365, their margins were horrible, like...I know I'm forgetting details a lot, maybe 6 or 9% first year, and less than half that recurring. We actually got asked to go up and meeting the early 365 partner reps in Hartford CT, to give our feedback.
Fast forward a few years, and as Rob notes above, there really is "so much to learn" in 365. I see so many people, including tech people, that think 365 is "just email". There really is a whole suite of services that all compliment each other. But my main point here is....there's a lot for us IT people to manage here. I was relieved at decommissioning on prem servers, but the mistake is to think 365 is less time consuming. Sure..no more worries about hardware failures of on prem servers, and no having to get up extra early or stay awake extra late to do those monthly updates and reboots and checking things. No worries about having to do restores from back up of a server. (granted...Datto made that SO easy). But we have so much to manage now. Security being one of them, MFA, and there's a lot of hand holding to go on here.
So long story short, I've mentioned this a few times in other threads, we still have a line item in our monthlies for our MSP clients, used to be for an on prem server, now we're going for that same price each month for "365 tenant management". Because we're in there often checking things. And..we're adding a 365 management tool soon, it ain't cheap. But it makes management of many 365 tenants easier for us, and helps push out templates to make sure clients tenants have all the proper settings dialed up for security 'n such.
Also, I encourage anyone here to REALLY set your standard to have clients on M365 Business Premium at a minimum. The added security features of Defender (previously called ATP...Advanced Threat Protection) are really needed. The benefits of "conditional access" are so worth having Azure P1. And the ability to have so many things "automated" via MEM/InTune are...SO worth it. It REALLY cuts down on YOUR time setting things up and managing the client. Thus...big savings in money. And big savings in your time spent. Making the client more profitable for you too if they're on a fixed monthly. The small cost per license over 365 Standard is easily repaid multiple times for both sides.