If you examine the US Navy's ship force levels
after the end of the Cold War you'll find that the navy downsized from 500+ to 300+. During the height of the Cuban Missile Crisis the navy had nearly 900 active ships. During WW2, 6000+.
So if you can imagine Starfleet having to deal with hostile Klingons for 70 years then going to a peace-time stance there is plenty of opportunity at that point to retire most of their military ships and downsize to just exploration and science oriented ships. With fewer ships you get the "overstretched and overreached" Starfleet in TNG trying to police their own space.
The other thing to also consider is with a changed Starfleet comes changed training priorities. As Riker put it in "Peak Performance", combat training isn't high on their priorities:
RIKER: I prefer brains over brawn as well. I think it's a waste of effort to test our combat skills. It's a minor province in the make-up of a starship captain.
And this is proven when Riker while in charge of the E-D gets defeated by two BOPs run by Ferengis. IIRC, even Odo makes fun of this.
But if you go back to the Klingons we know that they have very long lives. Kor, Kang and Koloth and all their generation of warriors would've been able to contribute to battling the Federation whereas Starfleet would be in a scramble to train up new warriors.
Starting with the longevity.
We know that humans aren't exactly the longest lived members of the Federation by a long chalk, with several species that live substantially longer lives. Vulcans at least live into their third century if they don't get sick with odd strange illnesses, and there's a bunch of other species that have longer lifespans. For that matter it's evidently not at all extraordinary for human Starfleet officers to be in service as centenarians. I don't think it's at all likely for them to have much trouble coming up with trained warriors, especially when there's several Starfleet member planets with strong military traditions like the Andorians.
Second, US Navy Force levels between the Cold War and modern USN aren't particularly helpful when you look at the composition of the ships employed. A modern DDG could wipe the floor with a squadron of Cuban Missile Crisis era frigates and destroyers, or cruisers for that matter. While downsizing was very real, if you note how the force composition changes the difference between 89 and the late 90s boils down to:
- Removing all the battleships from service.
- Decommissioning CGNs and CGs in favor of newer aegis equipped DDGs as their refueling cycles or SLEP cycles came up. (Decommissioning of Belknap and the 3 CGN classes that were well into their third decade of service and replacing them with more capable DDG-51s)
- Significant reduciton in the number of auxiliary ships.
- Reduction in number of lower performing ships like FFs (like the Knox-class) in favor of increasing proportions of more Burke-class DDGs.
In Starfleet terms it might mean that after the Khitomer Accords, Starfleet demobilized a significant portion of its less useful/flexible ships in favor of ramping up construction of more capable and flexible cruisers. This would coincide with a huge building program of Excelsior
class ships filling in for all the FASA and FJ auxiliaries and ships of dubious utility.
Also re: Rascals
, we know the scenes used for that episode were reuses of the Klingon K'vorts attacking the Enterprise
-D in Yesterday's Enterprise
, so perhaps those were the very big warship type BoPs retrofitted with more advanced weaponry. Combined with the fact that the klingons are ostensibly allies, and they were in the middle of a delicate rescue operation I'd imagine it's not really Riker's fault that they were caught flatfooted. We know at least that Riker's presence (and tactical acumen?) were considered so vital by Q that he claims no William Riker would have led to no Federation after Wolf 359, and he seemed to have no trouble outmaneuvering the So'na. (The latter granted after a bunch of years of war)
Further in hand to hand combat Starfleet officers seemed more than capable of holding their own against Klingons circa 2370 (Way of the Warrior
), so I don't think the "they let their military training lapse" thing really works as a reason.