I work for a government institution and I assure you that we have more than 20 vendors for IT.
If you replaced that Microsoft system right now you would have to find individual vendors for each of the parts that Microsoft provides. Getting them together would be a huge nightmare, because even the basics do not work.
The end user devices are Windows 11, we use M365, but government services are mostly homegrown and the infrastructure runs on Broadcom (VMWare) and IBM (Openshift) software.
Replacing Windows 11 with some kind of Linux and M365 by an MTA is technically feasible, there is political momentum building against US-centric services, but here in Europe politicians are historically highly suspicious of technicians, so nothing gets done yet.
It's a rich country, COTS replaced a lot of technical excellence, but the trend can be reversed as we have bright engineers on the inside still.
In poorer countries and regions, the engineering excellence is way better and they are much more independent.
The Microsoft servers are most likely azure running linux. Thats quite possible to replace by any number of vendors.
The main MOAT microsoft has are the contacts and the lobby. There always is some politician around fighting for Microsoft because they like Outlook more than Thunderbird.
It’s also reason why i think they will keep their dominant position. Even though the idea they provide something rare is increasingly more untrue.