The flute doesn't make for a good example, as the end user can take it and modify it as they wish, including third party parts.

If we force it: It would be if the manufacturer made it such that all (even third party) parts for These flutes can only be distributed through their store, and they use this restriction to force any third party to comply with additional requirements.

The key problem is isn't including third party parts, it is actively blocking the usage of third party parts, forcing additional rules (which affect existing markets, like payment processors) upon them, making use of control and market dominance to accomplish this.

The Microsoft case was, in my view, weaker than this case against Apple, but their significant market dominance in the desktop OS market made it such that it was deemed anti-competitive anyways. It probably did not help that web standards suffered greatly when MS was at the helm, and making a competitive compatible browser was nigh impossible: most websites were designed for IE, using IE specific tech, effectively locking users into using IE. Because all users were using IE, developing a website using different tech was effectively useless, as users would, for other websites, end up using IE anyways. As IE was effectively the Windows browser (ignoring the brief period for IE for Mac...), this effectively ensured the Windows dominance too. Note that, without market dominance, websites would not pander specifically to IE, and this specific tie-in would be much less problematic.

In the end, Google ended IE's reign by using Google Chrome, advertising it using the Google search engine's reach. But if Microsoft had locked down the OS, like Apple does, and required everything to go through their 'app store'. I don't doubt we would have ended up with a similar browser engine restriction that Apple has, with all browsers being effectively a wrapper around the exact same underlying browser.

Apple is dominant is their closed garden approach

I really don't think that's true, I think Apple became dominant through being first to market, having attractive design (was largely sold as a fashion/luxury item), and attracting devs early on (mostly through being first to market). Most of the value of the App Store was the quality of app reviews, which was due to developer fees (raise barrier to releasing trash) and actual app reviews, and that's how Apple earned their 30% cut. Since iPhones were a luxury item, they attracted people willing to actually spend money on apps, which attracted more developers.

I really can't see how not having other options somehow improves the attractiveness of iOS. Having high quality apps on the App Store made it more attractive, sure, but it didn't make other app stores unwanted, in fact not being able to side load apps/stores has been a complaint since pretty much the beginning.

Nobody is saying Apple is bad because they're popular, they're saying Apple is bad because they're anti-competitive.

I didn’t understand back then I don’t understand now why they lost lawsuit if they didn’t, IIRC, block you from installing anything else.

Microsoft restricted access to internal APIs that made the browser work a lot faster, so other browsers would always be slower and a worse experience vs Internet Explorer because Microsoft prevented them from getting the most out of the hardware.

You could install an alternative, sure, but it would be hamstrung and most would blame the browser, not MS.

Having a default wasn't the problem, Microsoft still has a default browser to this day and it's totally fine. Being anticompetitive, however, isn't fine.