"Lock-in" means that there are alternatives that you could use, but the vendor does not let you. When Metal appeared, there were no alternatives, Vulkan wasn't a thing for 2 years after Metal, and even today, Vulkan has seen relatively low adoption, so one could argue that it still isn't really a thing.
Metal might be many things, but lock-in isn't one of them. If you are happy with Vulkan then good for you - without Metal, Vulkan wouldn't even exist. It took Apple telling Khronos to GTFO for they to start working on Vulkan.
That's completely false. Apple only started the whole Metal effort because of Mantle, which was the origins of Vulkan. But being Apple, instead of collaborating and ending up in proposing things for Vulkan, they pushed for NIH lock-in.
> Lock-in" means that there are alternatives that you could use, but the vendor does not let you.
Which is exactly the case here. Take some project like Wine for example, that works on implementing Direct3D over Vulkan. That's the alternative they want to use, yet Apple prevents them, forcing them to either waste resources on implementing another path using Metal, or to go suboptimal route of extra translation. Who is to be blamed for it, but Apple?
> Metal might be many things, but lock-in isn't one of them.
It wouldn't be, if Vulkan was available as an alternative. But like you said yourself, lock-in happens when the the vendor does not let you use the alternatives. Who but Apple prevents native Vulkan support from appearing on their systems? So Metal is very clearly lock-in, according to your own statement.
You're understanding of the history of Mantle, Metal, and Vulkan is very poor.
1. Mantle was not originally being proposed as a "open" API as you are implying. A reading of the [original white paper](https://www.amd.com/Documents/Mantle_White_Paper.pdf) makes apparent that AMD was trying to leverage their game developer mind-share from the consoles by bringing it to desktops. Mantle was being proposed as an AMD GPU only API.
2. Metal was released 10 months after Mantle was announced (and 3 months after API docs surfaced). That's an extremely short timeline for a big company to go from zero to deciding to release a new graphics API, spec that API and it's shading language, write drivers and compilers for it, write developer tools, and author documentation. The timeline you're proposing simply doesn't make sense. Apple was clearly working on Metal long beforehand.
3. Vulkan, as an initiative, was reactionary to Apple's release of Metal, being that - up to that point - iOS and macOS were the largest OpenGL[ES] markets and Apple the only company commercially invested in OpenGL (OK, maybe not the only, but clearly Apple was the most influential Khronos member). By the summer of 2014, it was clear that Mantle - as an AMD only API - was not commercially viable. Thus, it was "donated" (so to speak) to Microsoft and Khronos to bootstrap DX12 and Vulkan development. (My understanding is that DX12 was already under development, but it's clear that the API shifted drastically in response to Mantle as evidence by the DX12 version 1.0 docs containing passages verbatim taken from the Mantle programming guide).
Please stop spreading false narratives in order to bolster your agenda.
It wasn't open from the start, but AMD communicated their intent to open it. No point to pretend Apple didn't know about it. It took a while, and AMD did just that, by giving Mantle over to Khronos, as a base of Vulkan.
They responded to it. AMD were first to come up with such design and published it for everyone in the form of Mantle. AMD also clearly communicated their idea to make it open and available for all.
Apple used the idea, but instead of eventually joining Vulkan group, pushed their NIH in their typical fashion.
Metal might be many things, but lock-in isn't one of them. If you are happy with Vulkan then good for you - without Metal, Vulkan wouldn't even exist. It took Apple telling Khronos to GTFO for they to start working on Vulkan.