In today's modern military, would there be any benefit to an enlisted structure similar to that of the past?

I'm going to go out on a limb and say yes. If you're happy at your present job and want to do it I don't think you should be forced to move up or out. Sadly some people aren't meant to lead, but worked their butts off where they are. No reason there shouldn't be lateral promotions.

If they did, I would love to become the SPC 9 of the army

But seriously, I don't see the benefit. As you increase your grade, your responsibilities and your influence expand quite a bit. You are often no longer just doing your MOS, but asked to take leadership in other areas. Thus, as a Soldier moves through the middle grades, they need that junior NCO leadership development to help them as succeed later. The closest we have to senior Specialist ranks are MSG and SGM. But, even though they are often in Staff positions and not command, no one will doubt their ability as a NCO leader

The question pops up from time to time.

The major issue is one of "force structure." For every Non-NCO in paygrades E4-E9, we must give up an NCO of equal grade. That's the drawback.

Why use someone who is not fit to lead when you get someone who is fit to lead (and capable of performing the same primary task) for the same cost? (both financial and opportunity)

If a person has a skill that is important enough to warrant the pay, shouldn't they also be expected to mentor others? In other words, to lead? Service in the military is transient and no one is irreplaceable unless there is no one to replace them...

As a soldier who was enlisted in that old rank structure, the problem was that all to often senior Specialists (SP6 and SP7) were often thrust into leadership positions and were untrained and woefully unprepared to do the job. As a Warrant Officer who is the technical expert in their field we are called to teach, coach, mentor and lead at all levels. As stated in other comments, as you progress through the levels your responsibilities expand. I see no sense in going back to a system that just didn't work.

I would say yes the older ranks could be useful. For example I knew a mechanic who couldn't lead you to the defac on time. However you gave him a vehicle that needed a repair, nor just a regular repair but the kind that stumps most mechanics, and you would watch a miracle happen. He would find and fix anything that was wrong with any vehicle. Why not promote people like this through the specialist ranks. To me I see it a the warrant officer in the enlisted ranks. An enlisted subject matter expert. It could also help with our NCO heavy army and be a step to balancing things out.

This has been beat to death in other posts. My background involved the Specialist ranks, in fact, I think I was the last SP6 on Fort McClellan while I was going through the transition course for Chemical NCO.
The reality of the Specialist ranks is that it was mostly tied up with a soldier's MOS. The exception to that was SP4. Almost everyone who made E4 (from early 70s at least) was a Specialist. I knew some who had to take a reduction to E4 after a break in service as an NCO and they were pinned as CPL. In technical MOSs like Satellite Communications everybody was a Specialist, even the leadership. And I am sure that there were guys wearing "hard stripes" in some Infantry units that were only in charge of their desks rather than soldiers.
Specialist ranks were not the ancestor of Warrant officers. The Warrants were around long before the specialist ranks came out, specialists were more an offshoot of the ranks with the T in the graphic - Technical Sergeants (from WWII era)

This would be more applicable as we build a "cyber" force. A lot of "techies" like to focus on the computer monitor, and not necessarily leading others. I would have no problem with a room ful of spec five, six and sevens cyber sleuths, reporting to a warrant officer. Think of the savings (cost, and billets) of not sending this group to NCO and related schools. I'm assuming these individuals would never be deployed into a combat zone.

While doing Army support duty as a member of the Air Force I became familiar with the idea of the specialist grades. I think this is something that all of the services could benefit from as there are many within each branch who do not want to promote themselves out of a job. I personally knew guys in the AF who would score low on promotion tests so that they could continue to do the job they really enjoyed instead of getting promoted into a job they did not want to do.

There’s many a job that takes knowledge and work but not leadership of more than 5 or 6 men. Cooks, finance, personnel, Signal, maintenance. Good men doing a good job but not NCO’s. They deserve to go up in rank on ability. I should have never been a Sgt.