For me it would be harder to gather the same know-how on closed systems, because you need your company to back your training on the tools you need to do a job, spend money on the licenses, jump tool when the vendors decide to discontinue a product, etc. Where I come from, if you work for a small company you’d be expected to learn as you go. Maybe things are better now, I don’t know.
In my opinion Linux (well, FOSS actually) gave me a great big box of small Lego^TM^ bricks and the freedom to build anything out of it. So I’ve worked with HW clusters, then virtualization was all the rage when CPUs gained more power, then containers, then container orchestration, then cloud… Complexity is increasing, but the knowledge I gained from knowing that in the end it is just a bunch of processes running on a Linux kernel makes learning the next big thing more manageable.