But, as soon as they encountered them, they took to them rapidly (look at e.g. Sengoku Japan, which quickly developed its own firearms industry).
And even if early firearms were crude, they will be subject to further development. They advanced rapidly OTL, after all, and not just in the form of handcannons, but also siege weaponry (which everyone recognized as a possible application right away), which will encourage continued development of firearms technology, even if personal weaponry is not initially emphasized.
Besides, even if you somehow prevent gunpowder from developing, crossbows and other ranged weapons are becoming increasingly important. The fact is, as soon as you get the ability to reliably kill someone before he can come close enough to kill you, ranged weapons are inevitably going to dominate. That's not really something you can change, it's simple logic.
First, projectile weapons are as old as melee weapons; they have coexisted from the dawn of warfare up to maybe the last century. The dominance of projectile weapons can be forestalled quite a few decades, perhaps indefinitely.
Second, good plate was very, very, very hard to penetrate even with period muskets, let alone a crossbow or arrow.
Quality plate was essentially immune to projectile weapons during the early modern period, just as mail was more or less invulnerable vs bladed weapons in the middle ages. It was logistics that killed the knight; but if I had to blame a single weapon for the rise of infantry I'd pick the pike, which offensively and defensively dominated infantry tactics for the early modern era, guns slowly becoming more prominent and effective over the centuries and eventually replacing pikes entirely in the 1700s. No guns means pikes stay dominant, as I doubt any handheld crossbow could match the power of even 17th century muskets while being reasonably man-portable. Hence plate armor can be lighter and more effective, horse armor also remains practical, and the armored knight can defy crossbows even more safely/easily than they could damn the muskets of pike and shot squares- I recall some expression that "arquebusiers without pikes are naked" and liable to be ridden down and slaughtered. Hence why the defensive formation was so important, and why we were using lines and squares into the American Civil War- cavalry were death to infantrymen in the open, unless they had a good formation of pikes to fend them off.
Pikemen are cheaper, faster and easier to recruit
en masse than a professional warrior caste. Polearms turn the cavalry charge on its head- the kinetic energy of the horse+rider goes into him via hitting a pointy stick; although horses broke the charge/stopped short of pike walls, they were smarter than their riders in that regard.
So essentially without the advanced guns of the 1500s onward the armored lancer retains his place of pride as the tank/shock troop of his era. Furthermore infantry armor stays around a lot longer. Pikes become king of the battlefield, with fully armored lancers retaining a small but extremely decisive shock role, and varying degrees archers, crossbows, and the odd two-handed swordsmen/poleaxe/halberd wielding armored lunatic filling a subsidiary/supporting role to the pike lines and squares.
Fortifications and naval combat are more heavily/immediately affected. Without cannons boarding likely sticks around longer- tacticians are VERY conservative, even in the 19th/20th century they stuck with ramming long after naval guns got big enough to put holes in ironclads at range. Sailing vessels probably still come about, as galley-style vessels while great for littoral waters/the Med have horrid logistic requirements and manpower overhead limiting their effectiveness for trade/long distance sailing. So Atlantic/Pacific/Indian Ocean navies use sailing vessels a la the cog, but no cannon means, other than boarding action... ballistae, perhaps? In any case you'd have a strong defensive mentality both land and sea, with castles remaining "medieval" and not getting the Star Fort treatment.
The other question is horse archers- do Mongol style armies benefit from the lack of guns? On the one hand plack powder was great for siege artillery/mining as well as smokescreens and signal flares and the like, all of which were used brilliantly by Mongol generals. OTOH, no guns means horse archers and cavalry in general aren't as outmatched versus ranged infantry. Heavy armor is expensive and lowers mobility; though "lighter" armors e.g. mail and lamellar are decent and somewhat prevalent in the hands of tactically brilliant horse archers I suspect only the most professional/well equipped and led Swiss pike block could fend off/win against a mongol style horde. Light cavalry becomes prevalent for raiding, skirmishing and tactical maneuvering versus infantry, as it was for the Eastern Europeans during this period and also for the west in later (1700s-Napoleonic) times.
In brief- tactics, terrain, training, organization, quality of soldier/gear/leaders, and the recklessness/discipline of your and the enemy forces will determine how pike v.s. knight v.s. Mongol v.s. French turns out, and there's quite a lot of leeway for how an alt-timeline general can fight and win, with whatever level of "projectiles" he has or is up against.