Of course the regular match goers are likely to still attend. But, in my experience, lower league/non league clubs don’t have a huge number of “die hard” fans. They rely a lot on floating supporters. So I think it’s fair to assume that their matchday attendance would reduce somewhat if, say, a Man City or Chelsea match was being shown live on TV at 3pm, just as their game was kicking off.
These floating fans, like myself, might be the difference between survival and bankruptcy for many clubs. The gap between the Premier League and lower league is already massive and this would just make it wider. The PL can literally show live matches at any other time, do they really need to take over that 3pm slot, too?!