The things you are pointing out are orthogonal to the architectural tactic of microservices...
1) RPC and remoting can leverage microservices, but are not required to. There is a very, very, low probability that people doing DCOM or SOAP were doing microservices. Modularity is an essential feature of microservices. Service experience does not necessarily translate to micro-service experience...
2) Is this GUI app service oriented? That is to say: despite its thick client, is it accessing network and shared resources in the networked environment? Because native GUI apps often talk with webservices. Maybe those are monolithic, maybe they're not, but the resource usage will have to do with development and maintenance of those service and its effect on the (fat) client.
3) Traditional three tier architectures struggle in numerous scenarios (particularly those where microservices thrive). Hexagonal architectures/DDD do well in those areas, provide all the same benefits and more, and are really not "cool" anymore. Also, those design styles can use microservices (or not), they're not dictated by them.
4) .Net Core 3 has a bunch of value adds for thick client WPF apps with no services -- side-by-side installations and such.
5) You should not ask your customers to waste money with microservices, nothing about .Net Core 3 would indicate that you should. Same deal for monolithic services... Just because MS launches something good for apples does not mean you should call your customers and tell them their oranges have to be thrown out.