Erm I disagree, quite often on Immortal or even Deity (sometimes) I've gone to war with my Riflemen vs. medeival unts, in fact this happens quite often with my playstyle.
I'm not trying to say I'm some 'pro' or anything, that's far from the truth, but my playstyle is to spend the first 1/2 of the game soley on bulding up my economy, mass-producing as many wonders as possible (which is usually lots), and tech trading aggressively (though carefully) with the AI's to ensure I have a good tech lead. It's more then possible on Immortal+ to usually have a decent tech-lead by the time you can build riflemen if you play your cards right with a leader who has economic traits. (though some games you may be stuck with bad terrain/etc making it harder)
What's the point of going to war vs superior units or even units at par with your military? I virtually always pick my time period for wars when I know I have a good tech lead, and often it means my riflemen vs their knights, then shortly after it's my infantry vs their rifles/cavarly.
At any difficulty this seems to apply. I usually play on Immortal and often after a few golden ages from Taj/great artist I'll advance to the industrial era and this is most often when I goto war, and 95% of the time I'm the tech leader at this point. I do usually play leaders with economic traits (like financial/ind/philosophical etc.) so maybe with Churchill it would be more common to find yourself against other riflemen with his UU, but with Victoria/Elizabith you should be able to have a decent tech lead and not need the +25% vs gunpowder IMO even on Immortal/Diety difficulty (if that's what you play).
It can depend on your playstyle I suppose, some players are much better warmongers then I am, and tend to goto war early w/out a tech lead and win, but I tend to focus 99% on my economy and later goto war when I'm far ahead as possible in tech.
Redcoats used to own with their superior strength, now I hardly find them anymore usefull then riflemen. (and its usually not that long after getting rifling that I have infantry anyways).