To attract a sizeable free-to-air audience for sponsors, Champions League coverage needs to be visible within the schedule/EPG of a mainstream free-to-air channel, on the viewers' platform of choice (all major platforms), in my view.
ITV were by far best placed to deliver this and I doubt that UEFA/its representatives were unaware that BT wouldn't/couldn't deliver free-to-air audiences remotely near to ITV when choosing to accept a huge increase in rights payments from BT.
Due to viewing behaviour, neither BT or Sky could deliver a free-to-air audience remotely close to ITV. If the Champions League isn't on ITV, then ITV are competing for audience against it!
If UEFA regard high free-to-air audiences to be more important than maximising rights revenue, then they'll only achieve this by not selling exclusively to a pay-tv provider again, and guaranteeing free-to-air coverage on one of the major free-to-air channels.
However, (per mlt11 earlier in the thread), UEFA have previously indicated that the revenue split was approximately 75% TV : 25% sponsorship.
So, in economic terms, tv revenues are significantly more important to UEFA than sponsorship.
In my view, UEFA's biggest concern will be seeking to maintain TV rights revenue at the level currently paid by BT and they may have to accept an exclusively "pay-tv" deal next time, to achieve that.