The threat in the North Atlantic is at an all time high since the dissolution of the USSR. With a steady production schedule for the Yasen-Ms and diversified submarine fleet ranging from smaller Lada and Kilo SSKs to the variety of SSNs and SSGNs like the Yasen family, the Oscars and Akulas. Not to mention that the northern fleet alone matches the SSBN presence of the entirety of Europe, with the Borei-A's being currently the most modern ballistic missile submarines in active service. Add to that steady modernization in form of new equipment and especially weapons integration, as well as several ongoing programs for UUVs and XLUUVs, the submarine threat is at it's highest since soviet times. The threat level in the Pacific Ocean is higher, because the PLAN is completely dominating regional naval forces, but on the other side there is a considerable USN presence and the JMSDF and ROKN make any european navy look completely inadequate, except perhaps the French Navy (although certainly lacking behind in the realm of surface combatants compared to East Asian fleets). So the threat level in the Pacific is higher, but all forces involved are quite threatening. While in the Northern Atlantic the truly threatening fleets are the French and the Russians, in litoral zones the Norwegians and Swedes too, but not in the open ocean. When the sum of several entire naval fleets is equal to one portion of one Navy, I wouldn't call the threat environment low, let alone shrinking. Especially as Russia could simply re-deploy portions of the Pacific fleet into the area of operations of the northern fleet.
Big difference, in favor of the Russians, is that Western European naval forces have melted away in terms of numbers and the Royal Navy in particular is significantly weaker compared to what it was during the height of the cold war. Their premier ASW asset is the Type 23 as we speak, this should say it all. Then you have premier surface combatants not even close to approaching a dozen in numbers and certainly not in active duty and the picture is rather clear.
The Russian Navy is strengthening it's position in the Northern Atlantic and Arctic, especially in the underwater domain where it already enjoyed a considerable lead. While the large Naval Forces of Europe either stagnated or have grown weaker over the last decades. Pair that with continuously tremendous investment into the development and upgrades of the anti-ship missile arsenal, unmanned underwater vehicles and the integration of new weapons into their existing vessels, it's not looking rosy. Especially as the most capable European Navy's, the French and Italian fleets, are mostly active around the Mediterranean. And they are not hull count wonders either, although at least France has always tried to remain competitive under the sea and in the missile domain, unlike the UK.
To, appropriately for this thread, frown at the Royal Navy a bit more, when your most capable assets are 9 foreign procured, airliner based, MPAs which are super susceptible to ship based SAM and fighter aircraft, are you at this point even trying to pretend to be a capable sovereign Navy in North Western Europe? Because given the failure of the QEs to be anything resembling useful, the failure to get them decked out with the fighters they meant to carry and which are adapted for the compromised design of the carriers, when your premier AAW destroyers are derelict and suffering from low availability, when your premier ASW frigate is a design of the 1970s, when you can't manage to get the budget you need to stand up a capable Navy and basically have to rely on allies these days to protect your waters, when your SSNs are in a terrible state as well. Then are you even a serious consideration in the power dynamics of the Northern Atlantic?
When all you can bring realistically to the table are 1 QE carrier with ~ 12 F-35Bs, 1 to 2 Type 45s, 2 to 4 frigates, 1 to 2 Astutes, your P-8s operating from land (thus a target for submarine launched missiles), Merlins and that's about it, then you are clearly not a Naval Power at this point. The JMSDF or ROKN could realistically contain the Northern Fleet in 2026, if they were to swap geography with the UK and other Western European countries. But the RN would find itself at the receiving end of a Kalibr, Oniks, Granit and Zircon turkey shoot. And they don't have the numbers to absorb such losses. The advantage lies with fixed wing ASW, the P-8 being the backbone, but that doesn't off-set major shortcomings elsewhere, especially in the subsurface and missile realm. And while the VMF fields around 25 Su-30SM/SM2 from land, it's known that the VKS also conducts patrols and escorts, and as soon as Su-35s, MiG-31s, Su-57s, more Su-30SMs, Tu-22M3s and others enter the equation, it becomes more and more questionable just how survivable North Atlantic aviation and MPAs actually are.
So your assumption is ultimately the exact opposite of what's the current reality. Which is that the threat environment is currently at an all time high since the cold war and that, while others are stepping up to compete in that environment (like France, Norway, Sweden, Germany), the Royal Navy in particular and other formerly more important players like the Dutch Navy are dropping the ball extremely hard.
