And see, THAT is the problem. The early firearm units in the game should not be "redundant."
In real history, the
first handheld gunpowder weapons altered the nature of warfare from quite early in their introduction,
even before muskets. It did not take "rifling" for firearms weapons to become good for something besides defending!
Moreover, from the 1400s through the present, there has been an
ongoing evolutionary process of gradual changes and improvements in firearms. While admittedly the game cannot represent every little improvement, at present it skips over a huge amount of change in two steps Musket-Rifle-Infantry. There should be MORE steps!

and maybe Rusty Edges' inclination for their to be more units and Techs is the real solution.
At minimum, I'd think something like this:
(1) First arquebus matchlocks ~1450
(note
"True rifling dates from the mid-15th century, although the precision required for its effective manufacture kept it out of the hands of infantrymen for another three and a half centuries"; so instead of having a "Rifling" Tech, it should be something like "Inexpensive Mass Produced Metals" Tech!

)
(2) Replacement of arquebus by musket ~1550
(skip various improvements in musket: wheel-locks, snaplocks, snapchance, etc.)
(3) Replacement of musket by flintlock ~1650
(4) Replacement of standard muzzle-loader ammo by
minie-ball technology ~1840
(5) First "Rifles" (better seals, cartridge bullets, breech-loading, better rifling, etc.) ~1860
(6) Efficient and cost-effective "machine guns" ~1900
(7) "Modern" rifle (long-range, high-reliability, high-durability, high-accuracy, bolt-action rifles, telescopic sights) ~1900
(8) Assault rifle ~1935??
(9) Mech Infantry . . .
Why there is a need for an entirely separate "unit" called
"Grenadiers" in this game, which retains its effectiveness for not just a short span of the 18th century as a counter to early muskets, but as a counter to RIFLES! all the way until infantry I have never understood. It is in total contrast to my understanding of the actual history of warfare. I tend to think Firaxis did not make sufficient use of military historical consulting in their formulation of the late Renaissance to early industrial Techs. Indeed, even the Infantryman is too simplifed IMHO.
Again, I'm no expert, so I defer to the perhaps more precise knowledge of other folks, but my understanding is that firearms had made "the push of pikes" an obsolete way to wage war by about 1600? Thus, "redundant" is the LAST thing that muskets should be.
The first breech loaders did not gain real dominance until the 1860s, and by end of ACW, most muskets had been replaced by breech-loaders, i.e., what is referred to in the game by the "rifling" tech. That is roughly 250 years during which the unit "Musket" _should_ be playing the increasingly dominant role in the game. Grenadiers should be a counter to Muskets, and only on the attack, and ONLY up until some new Tech that makes them less effective.
I think that the basic way the musket fits in the game, being only a strength 9 versus a strength 8 "maceman/man-at-arms" is the real problem.