From their FAQ https://docs.modular.com/mojo/faq.html

"Will Mojo be open-sourced?

Yes, we expect that Mojo will be open-sourced. However, Mojo is still young, so we will continue to incubate it within Modular until more of its internal architecture is fleshed out. We don’t have an established plan yet.

Why not develop Mojo in the open from the beginning?

Mojo is a big project and has several architectural differences from previous languages. We believe a tight-knit group of engineers with a common vision can move faster than a community effort. This development approach is also well-established from other projects that are now open source (such as LLVM, Clang, Swift, MLIR, etc.)."

The problem with this is that you don't know what the licensing will be when/if they open source it. So you could start using it now and they might change their mind and choose not to open source it or it might be a restrictive license.

Also, even if fully open source, have to also look at governance.

(A canonical project can be steered in a way that makes it impractical for you, and forking is often also impractical.)

For now, I'd treat it as closed source, which is a non-starter for investing in, when I can accomplish the same in open source ways. And there's no sense in giving away the open source uptake benefits to a company when the software isn't open source.

That's true, but the whole language is unstable, so you shouldn't use it for anything big anyway.

Their response to why it's faster to move while closed source seems kinda like a false dichotomy. They can choose to start with it open disallow input from the community or close their issue tracker to the community.

Why bother doing it that way? Just to entertain people who works like to observe the commit history as the language evolves?

It gives people confidence to start working with it now.

I don't want to commit to a language/ecosystem that might pivot and not open-source.

So that people trying out the language have the freedom know what they're running on their computer.

It's closed so others don't catch up quicker or add those optimizations upstream? Then they have no moat.

So what's the business model then?

Do programming languages need business models?

One that pays expensive senior full time SWEs to build it does, yes. Every programming language uses one of two models:

1. Patronage

2. Business

Patronage is the Ruby/Python/Linux "one guy + volunteers" model where a few of the devs look for companies to pay them to do it as a full or side project basically for marketing purposes or because that company can afford to subsidize their tools. This clearly isn't that.

A VC backed startup making an open source language is neither patronage nor a business, unless you take the hard-cynic position of saying the investors have been tricked into being patrons.

I'm assuming they've got some sort of cloud related ideas for monetization, but it's tough.