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

Yes also I'm a graduate from Anco and Bnco it made me a better leader and a teacher to my charges my troopers

There was also a Platoon Sergeant (PSG) at E7 level.

Would there ever be a Spec9 of The Army?

My Dad was Tech5 during WWII. He was not a combat troop but a technician. He was a "true" Armorer.
He retired as the Chief of Inspection at Robins Air Force Base in Warner Robins GA in 1979.
He always said that he was best suited for that post as he knew the system of the Aircraft and other systems he would oversee.
There are soldiers that are leaders and those that are technically orientated. Those technically orientated soldiers need to have that avenue if advance in their careers.

No

Yes

Yes. Spc5 pay 1/3 difference between E4-E5. Spc6 pay 2/3 difference. Spc7 pay slightly below E5. Spc8 slightly above E5.

You have soldiers who can do a decent job, for whatever reason they are not able to lead.

Do not give them as many responsibilities nor as much pay.

I agree I was in under old system I think it is a better system

SP8 & SP9 I'm pretty sure was authorized but never awarded During it's heyday most spec.jobs were clerical,commo, fire control, computers, etc... personnel that did not need an NCO giving every next move that was necessary to complete task (in theory) NOT that they were not good enough to lead.

It would definitely benefit retention in units where soldiers can not financially support there family at E-4 and there are limited promotions in there MOS. Specialist 5 - specialist 9 would also make a convenient step program to becoming a warrant officer as an alternate to WOC school in some MOS’s.