Is an open-source piece of software more extensible than closed-source? If so, then if the goal is to maximize extensibility, then open-source is required.
Open source software is readily and legally user modifiable. It is often (but not always) readily extensible - it can actually be quite difficult to extend in some cases. (There's plenty of difficult to work with spaghetti on GitHub.)
Source available proprietary software is readily inspectable, but often not legal to modify (at least not for free). It might still be extensible (ex plugins) though!
Closed source proprietary software is difficult to inspect, difficult to modify, and generally illegal to modify (for most use cases, in most jurisdictions). As above, it could easily be extensible though.
Hardly. Extending a piece of software requires generally extensive familiarization with the codebase, possibly ranging into millions of lines.
On the other hand, a well documented plugin or scripting system, which sits on top of the existing domain logic and is well documented and full of examples, generally is an excellent way to allow extending the base app.
The base app can be open or closed source. Without scripting or plugin system it's still a black box for most intents (as the time needed to study and change it would likely be too much).
someone created something that they'd like to make a little money off of, while still taking very extensive steps to make it available, extensible, and prevent lock in. they chose a path to profit with all of these goals in mind. you're demanding an awful lot for something that "improves extensibility by even 0.0000001%". in the spectrum between "i want to extract money from the users of this product" and "i want this product to be used as the users see fit", i would say this product's goals lean towards the latter.
please don't say the wording is the problem. "as extensible as possible" is fine. "as possible" is a qualifier, it clearly means "as extensible as possible without undermining other goals". you're pretending that they've promised to make it extensible at the expense of _everything_ else, and that assumption in context in unfounded.
it's true that paths to monetization exist for open source software, but they usually aren't accessible to an individual developer who is building a small bootstrapped side-project to generate small amounts of passive income.
setting that aside, though, the best way to make software (open- or closed-source) extensible is via a plugin API. without it, the only way for end users to extend open-source software is to fork and maintain the fork forever, or attempt to merge upstream. so one might argue that the quality of the plugin API (and its documentation) is the primary measure of a software's "extensibility".
