Cannot see that rule happening considering that pretty much all the top clubs won't be able to satisfy the requirements (or go near it) and it will further affect English teams performances in UCL, making an UCL win completely impossible.the FA is trying to make it 12 players (and you have to play for 3 years before turning 18 to qualify as homegrown - if that change comes in then we will struggle!)
As of now regulars in the first team squad
Johnson, Smalling, Jones, Shaw, Carrick, Young, Lingard, Rooney, Rashford,
If the changes dont come in then we are still going to need some english players given that over the next few years certainly carrick, probably young and potentially rooney / lingard may not be here - throw jones in the mix and we struggle - if the FA do make it 12 homegrown players and change the homegrown requirements then we will be paying (even more) crazy premiums for english players like jones
Also, within the current rules, you have Schneiderlin as homegrown too, Januzaj, Pereira, Wilson and CBJ might return, TMF will be homegrown in the future etc etc. So not a problem at all to replace Rooney, Young, Carrick and Jones when the time comes.
If the new rules happen it will be a problem, but we will know about them a few years in advance, and anyway, it is quite unlikely that they are going to happen.