Think his talent will be wasted in Ligue 1 -- it's much better for the game if Neymar is playing in ~5 games per year against Madrid, plus a handful more against Atletico and another ~15 against the (still technically sound) also-rans rounding out the La Liga top 10. 5 goals every other week against
Championship / League One level teams at the bottom of the French league would grow tiring very quickly.
As for the (quasi-)direct impact on United, I'd actually still prefer Coutinho stay in the PL, as I dislike the trend of top players "stepping up" to Barca / Real more than I do Liverpool and their fans. The fact that Gareth Bale, the greatest player from the UK of his generation (and one of the best ever if you don't control for era), plays in Spain, annoys me greatly. You didn't see Shearer, Giggs, Scholes, etc. in a Barca kit, for chrissake. Having the Coutinhos of the world in the PL makes the league much more compelling, even if they're wearing a rival's colors.
It'd still be a short term boon, of course -- hugely damaging to Liverpool if the proceeds from a prospective Neymar sale were to be spent in that manner. Coutinho slotting a through ball to Suarez at Camp Nou would grind a few scouser gears as well.