But that's what the FA wants. They want clubs to play British (specifically English) players and rely less on foreigners. Brexit may help them accomplish that. But at what cost to the Premier League's quality and overseas marketability?
I don't think it's too bad.
If you have the right eduaction in academies, the standard of british footballers will go up. Germany has to counteract on it now, 'cause they only educated pass-and-move players with too much emphasis on systemic role play. Now there's not enough technical individualists. If you can do both with great educational work, it will only help the domestic talent. But maybe a reform of the youth league system is needed to align it to the German and Spanish model, where these are proper leagues.
It might actually help getting rid of that silly British Bonus you have to overpay for lads with domestic passports.
The rate of foreign players won't drop from 100 to zero instantly and there will be enough foreign top players to keep the league's "exotic" and intl flavour alive regarding marketability. I think by now this has brokenthrougha certain threshold and e.g. the US interest in the PL won't fade below a certain point - although Spain seems to be on the move over the big pond.
Also, there's enough money in the league to keep on luring foreign players with big wages. Teams will have more Chambers, Welbecks and Holdings to fill squad places, but the first XIs will still be studded with mostly foreign stars.
The only big point is see is the actual promise of success. Taking Germany as an example, around the time before and right up to the all German CL final in 2013, the intl success of not only Bayern and risen again Dortmund raised the Bundesliga's profile, but also the relative success of smaller clubs like Gladbach, Leverkusen, et al who excited on the intl stage with state of the art systemic gegenpressing game. That did lure more foreign players to the league. For a lack of actual winning something success, along a drop off of Dortmund, the league going stale again, the heightened profile couldn't be sustained and clubs thought they had top notch academies so they actually also stopped buying. I already laid out how that didn't work out as planned. Now there's a lack of "exotic" ball artists and along the missing success and the lost feature of exciting football the attractiveness has very much fallen.
For the PL that means money is great, but english clubs should start winning stuff again rather sooner than later or Italy or France might overtake them in terms of football attractiveness and the probability of success; also with City emerging as a club even more financially outstanding and willing to spend than last decades' oligarchs, the PL could very well become a one horse race down the line. English football might suffer a similar fate to Germany right now, with top players choosing other leagues and Spain taking over the US market. But that's more a mid- to longterm problem. Right now the attractiveness is still very much given. All in all I don't think new homegrown rules will affect clubs all too much, certainly not shortterm.