(I typed out an answer before, but it seems like it wasn't posted)

do you believe in OSM as a project?

Yes, definitively. It is widely used and known, many successful companies are alive based on the data and it is getting known.

What do you think its long term goal should be or even is?

To collect and publish geodata about the world - or at least about all somewhat-permanent objects that are located on this world. And we are succeeding at that pretty much!

Completely replace something like Google Maps?

It depends on what you precisely mean with "Google Maps". Google Maps is, first and foremost, an advertisement vector, disguised as a map. For many consumers, it is a map service which has lots of information about shops (including reviews and pictures) + navigation with traffic information.

So, some OSM-based apps might replace some of the services Google Maps offers, but many things (reviews, traffic information and pictures) are clearly out of scope for OSM (but can be filled in by Mangrove.reviews, mapillary or geovisio).

Having everyone use it as the main platform for looking at a map?

This is not the goal of OSM and should not be it. Have every map-app being based on OSM-data would be nice though. It'll bring other problems though, such as fighting spam and fake shops...

I sometimes wonder if OSS projects should have aims that ambitious.

Eh, it depends on how the OSS-project is structured. Is there an organisation behind it that lives from it and can make money of it, to fund it? Is it a hobby project? The aims and ambitions are to be decided by the project maintainers and all people involved. Many OSS-projects and volunteer organisations can be thought of as being a do-ocracy. Hackerspaces are a good example of that, and a good intro to the concept can be found here: https://hackerspace.design/

I run MapComplete a bit the same. I do the stuff that I either want to make or where I can get funding for in some way (thanks NlNet!). If someone passes by with a feature request, my reply often is that I don't have time, but feel free to make it yourself.

I can’t imagine a world in which people would all be using open and free standards, and I think it’s because I’ve become so accustomed to the commercial aspect of developing software as things currently stand. What I think I might be asking for is actually your overview and opinion on OSM as a project and what we could expect of it, going forward.

In my opinion, Open Source grows at a slower pace, but is more sustainable (or the project dies and gets forgotten). On the other hand, there are - for some pieces of software - incentives by the closed-source projects - to switch people over, to keep the money flowing, so it is an uphill battle.