Well that is going to fundamentally change the nature of the whole arrangement. A Ph.D. is not a work-for-hire kind of arrangement, it's more like an apprenticeship. If you want it to be work-for-hire then some other changes will need to be made.

When I hire a grad student I do so with the intention that I will keep them on for the duration of their education, which is a commitment of at least 4 years, sometimes up to 10 (do any employers have 10 year plans with you specifically in mind? No, as a 22 year old fresh out of college, you are fungible to them.) The bonds that are created between advisor and student can be one of the longest lasting and most important professional bonds a researcher makes in their whole life. It's so important that academic genealogy is actually tracked and recorded. There's a concept of a "grand advisor" or your advisor's Ph.D. advisor. So for all the negative stories you hear about a narcissistic asshole advisor wielding power and abusing underlings, there are far more stories of amazing intellectual relationships that can take a decade to fully bloom.

Your proposal would serve to virtually eliminate this relationship. The employee/employer relationship looks nothing like this. It's transactional and makes employees seem more like mercenaries than team members, no matter how much corporate culture wants to convince you that you're part of the team and a member of the family. You can actually find that environment during a Ph.D.; I know I did. You can make mistakes (sometimes huge mistakes) and not be fired. You can admit you don't know something or that you lack some skill and that's okay -- it's expected that you are still learning. Professors are happy to spend the time to teach such skills because they know they will be used on their own project, so it's worth while.

Here is what your proposal would do to my hiring practices and the way I run a lab: If you want to be an employee with the freedom to quit at anytime and move from lab to lab, I will conversely be quicker to fire you and to hire other better students from other labs to take your place. If you are going slow on a paper and you miss a deadline, I'm not going to shrug it off and say "Oh well, on to the next". I will probably fire you and hire someone else who won't miss deadlines.

If you make a huge mistake, like frying a $30k piece of equipment (true story), you'll be fired instead of met with understanding that you're just a student. Because that's what happens when you shift more onto the employee spectrum.

It's definitely also going to tighten the market for Ph.D. students significantly. This could be a good thing depending on your perspective, it would give you less competition. But it would also give new students fewer opportunities. I would think long and hard before hiring a grad student as opposed to research staff. If you make Ph.D. students too expensive, you'll just price them out of the market. Part of the deal of being a Ph.D. student is there's a general recognition that they will be working on projects they are not really qualified to work on yet.

As a 22 year old undergrad, you don't know enough about the domain to be useful. I need to invest years into you before you're even going to be publishing papers; if you can just leave me after that training to work on another project, I see no reason to invest that time in you in the first place. Instead, I'll just hire staff researchers who know what they're doing. Yeah they're more expensive monetarily than a grad student, but they also don't take up all of my time and make stupid mistakes that burn out expensive and one-of-a-kind electronics.

All in all, I believe your proposal is really throwing out the Ph.D. babies with the asshole professor bathwater. Honestly I joined academia to escape corporate hell, so as far as I'm concerned I'm going to keep treating my students as students rather than employees.