Depends on the sponsorship model. It's great to have 50million people watching a race but not many of those are buying the sponsor products. So do you charge for TV, or give it away for free and hope you make money later.
The real person that probably loses out is the tracks. A year's subscription to Sky Sports is about equal to a Brit GP ticket, no?
When a sponsor pays to advertise with FOM or with an F1 team, the money they expect to make is "maybe money". When F1 trades smaller audiences for increased television revenue, the fall in sponsor income is as predictable as the viewing figures - the more people are watching, the more sponsorship money you'll be able to attract. The only other factor that may play into the equation is the type of audience you're attracting, as potential advertisers will look at the demographics of the viewers as well as the outright numbers. So if you're only losing the viewers with the least amount of disposible income, for example, then it might not be as bad as it looks.
Nonetheless, if you reduce the size of the audience then you will, to some extent, reduce sponsorship and advertising income both for FOM and for the teams, as sure as night follows day. And the sad thing is that FOM only cares about its own loss of advertising income, so as long as the extra television money is enough to offset that, they will go ahead and do it even if, when you factor in the effect of the teams, it isn't worth it from the point of view of the sport as a whole. So you don't necessarily get the decision that is best for the sport, you could end up with a decision which is worse for the sport but better for the short-run interests of CVC's investors.
That's why FOTA, when it was united and strong, managed to get an agreement which limited Bernie's ability to put F1 behind a paywall in key markets, which is why RTL retains non-exclusive live coverage in Germany, and BBC retains either non-exclusive live coverage or extended highlights in the UK, etc. Now that FOTA is divided and weak, it's a rather worrying state of affairs, to be honest.
By reducing viewers you're also ignoring the long-run benefits of having extra viewers - ten year old boys are unlikely to watch an F1 race and then go out and buy a bottle of Shell motor oil or open a current account with Santander or buy a Mercedes, but if you can get them watching, you may have them hooked on the sport for the next 50 years. If you put the sport on a channel they can't watch, that's not going to happen.