There is a huge list of very negative effects, but the one most common one is that the ''effective range'' of engagement will not change for most ships.
This is due to the fact that many ships have enough agility to avoid shots from an X range. In terms of capital ships in parituclar such change would screw everything over without also rebalancing projectile speed in the process.
For example, the only thing that prevents gunboats from easily soloing a larger ship (read ''easier'') is due to their relatively shorter weapon range compared to cruisers and battleships. This puts them at -some sort- of risk as they need to get closer to their target. The larger ships might have better range, but due to their slower projectiles - they would not be able to hit those gunboats anyway. It would be a huge buff to all ships relying on speed-tanking.
You cannot fix this buff by adjusting projectile speed as increasing it for the largest ships would outright ruin anything small that comes close to them by accident or lead in close by hugging enemies.
Increasing range only helps against ships that are larger than the ones that they are being shot by, in short.
EDIT: Increasing the lifetime of projectiles also by extension increases server strain.