Is or has anyone argued against the importance of the merlin? Rhetorical question that, you know the answer
In your scenario, there would/could have been...
No Swordfish (or a massively compromised one with a less developed engine). Highly likely no sinking of Bismark. No crippling of the Italian fleet at Taranto... and thousands of unsunk ships, and a lot of detected, deflected or unsunk subs too.... oh and no 'woolworth' carriers as there'd be nothing to fly off them - and a big loss of anti-sub capability for the convoys later in the war. No torpedo bomber at all until the arrival of the Avenger and/or the awful Barracuda if you'll allow that to slip through on the grounds that its powered by the holy Merlin.
In fact, a world without 'the crap' leaves the FAA with nothing in the early stages until the arrival of the Sea Hurricane. So, no Sea Gladiator, no Skua, No Albacore, and no Fulmar. Without the latter, you massively up the chances of Malta falling too, as it provided most of the aircover in the Mediterranean in the early years of war
No Wellington or Stirling or Hampden. So no early bomber campaign - apart from the most vulnerable of all of the early heavies, the Whitley, if you'll excuse it on the basis that it has merlin engines. So that puts back both the campaign and the development of technology and technique by at least two years. It also gives the Germans the opportunity to divert significant numbers of aircraft, manpower and munitions to other fronts for a critical couple of years. (The Vickers Wellington with Bristol Hercules engines was the *most* produced UK bomber in WW2 - 600 of the bombers in the Thousand Bomber raid on Cologne were Wellingtons...) Wellingtons and Hampdens were also very useful torpedo bombers later in the war. Again, the Wellingtons were critical in helping cutting Rommel's supplies across the med.
No unsung but vital early transports - no Harrow, no Bombay, no converted Stirlings or Halifaxes and no Albermarle. So no glider tugs for DDay either. Nowt to see until the DC3 starts to arrive in significant numbers.
Coastal command has to basically give up on tackling the subs and protecting convoys - no (or massively compromised) Sunderland, no Anson, no maritime Whitleys or Halifaxes... All they've got left is a handful of Hudsons and Catalinas and a thin dribble of Liberators arriving piecemeal later on. The impact on the Atlantic war is probably going to be critical. Hows Harris going to react for a request for Lancasters...
Post DDay, no developed Typhoon with the sabre issues largely cured. So thats a massive compromise to the fighter-bomber campaign. What are the RAF going to do instead? Labour on with totally obsolete Hurricanes? For all its strengths and the fact that it was used occasionally as a fighter bomber, the spit is neither long ranged or rugged enough to entirely suit these operations - neither was it successfully adapted to take 8 x 60lb rockets.... You also lose the Tempest, a useful late war fighter bomber, and also the most successful aircraft for intercepting the V1s...
No Blenheim and no Beaufighter, so there's no early development of AI and technique for the nightfighters beyond the Defiant and Havoc
Why was the 'obsolete' Hurricane being produced until 1944? Because it was not obsolete in the far east and Burma; its construction and performance suited the low tech, harsh and spartan environment there - besides which, there were not spare more advanced types to be had
.... I think I could carry on in this vein for sometime...
The fact was that Merlin production and development WAS prioritised - lots of other engines were side-lined or abandoned; Peregrine for RR, Taurus for Bristol etc. The same with scores of designs and prototype airframes too - not because they weren't promising, but because the Ministries at least had a strategy for production prioritisation. You also seen to be ignoring that the manufacturers you mention WERE subcontracted to produce components and parts for many other aircraft than their on types anyway.
Going b@lls out for spitfire and merlin production leaves a small island well protected from a by a mid-war pretty much non-existent German bomber force (that would only have operated by night anyway), but pretty much unable to conduct a global war in all theatres. It also ignores hindsight - no one realised how well the Spit was going to be able to cope with being continually updated and improved.
Anyhoo, thats my take.