I think you are misinterpreting the meaning of the statement. Microsoft now has the ability to alter an OS at will. They can maintain Windows 10 and upgrade it without having to change the OS designation. If future upgrades are free then everyone will have the same OS. If future upgrades and not free, that might require some type of designation to keep them separate for updates, but only time will tell because Microsoft certainly will not divulge their plans.
Other revenue streams will take the place of the one which has been generated in the past by selling a new OS version. Therefore, Windows 10 will be the last designation... maybe. Whether those streams come in the form of online advertising and Store sales and such, or fees to receive upgrades, are options from what I can tell.