*shrug* Its been my experience too. I recently played a Brit/Otto vs Brit/Otto game. Their Brit massed Lbows and I massed Muskets and easily came out on top (that wasnt the only factor, but it was a trend I noticed when the units would fight it out)
I think part of the problem that people see is that its hard to see just HOW MANY Lbows you are engaging. Earlier in that game, I was building some Hussars (about 12) and sent them to engage a group of what looked to be about 20 LBows or so. I was slaughtered. I though that was ridiculous to lose like that. Upon watching the replay, those "20" Lbows were actually 37! They are just small, have a small footprint, and mass up really well.
In anything approaching equal cost, Muskets seem to do the job. Muskets also have more upgrades available at the AA and there are plenty of cards that affect Muskets.
LBows will beat Muskets if they are micro'ed and split up and moved out of range etc. So in Colonial/Fortress when there is more time for micro, Muskets usually will lose. But later in Industrial when huge piles of troops are present and cannon are present in quantity, I think Muskets are just fine vs Lbows.
As mentioned above, if players are still heavily micro'ing their LBows at 2200+, then yeah, I suppose Muskets will still likely lose, but for the vast majority of people whining about LBows, I just dont think that level of micro is being used.