But time IS arbitrary. It's not false, it's a fact. That's like saying anyone can learn to speak fluent French in a year. Send someone who understands language to live in France, and they'll be speaking French in a fortnight, and fluent in 2 months. After a year, they've changed their name to Jean-Pierre and own a small garlic farm in Rouen. Sit someone who doesn't understand language in front of a YouTube video about how to learn French, let them practice in their own living room for an hour a night... 2 months later they're lucky if they're any further down the line. After a year, they're frustrated and wondering if French is the right language for them; maybe watching a YouTube video about how to learn Russian!
Everyone learns, and acquires skills, at a different pace, depending on how much time they can dedicate to the subject, and how analytically and methodically they approach it. Why should cubing be any different? It's entirely arbitrary to say that everyone should be able to achieve sub 'x' within 'y' months.
I'd agree with your comment about "decent practice" though, and I think this is the key. You can do 1000 timed solves a day, but if you aren't analysing your own solves, just blindly practicing, progress will be slow. So it's just as arbitrary to say 60,000 hours of practice will get you to sub 'x'. There's a massive difference between good practice and bad practice, and bad practice may get you to sub-30, but not very quickly!
I'm guilty of it myself - I'm sure I could be sub 20 already if I'd drilled PLLs and worked on my lookahead with slow solves, but I just love solving the cube! I have limited time to practice, so I spend the majority just doing solves. I'm happy with my progress - I know sub-20 will come, because I know what I need to work on. But I'm in no rush!
How long it will take each individual to get to sub-30 it's impossible to say. For some it'll be weeks, for some it'll be months or even years. Some may never make it. With the right guidance, "decent practice", support from other forum members and good use of the available resources, it should be achievable for most, but putting a time limit on it only flatters those who are lucky or talented enough to progress more quickly, and frustrates those who are already struggling.