I did watch that FW video, but I have to disagree about the viability of a WW1 SLR, especially given that multiple automatic rifles using the systems developed pre-WW1 in the French SLR program were produced; the Chauchat used the same system as the Meunier SLR rifle and was close to the weight of the ENT B1 automatic rifle, despite being more complex. SLRs existed pre-WW1 as well, like the Winchester 1905 (and others):
https://en.wikipedia.org/wiki/Winchester_Model_1905
Which BTW actually was used in WW1 by multiple nations.
The only reason the French didn't actually adopt any of their SLR projects pre-WW1 is that their program was a mess of 'overdevelopment', meaning they kept changing specs, calibers, and doctrine, which meant that when they finally were able to settle on a new caliber and SLR it was already 1914 and too late to mass produce it, so they spent a few more years to turn out the FA 1917/1918 using much of the existing Lebel rifle parts and the 8mm Lebel cartridge:
https://en.wikipedia.org/wiki/Fusil_Automatique_Modèle_1917
So SLRs were used in WW1 and were successful; they were also viable pre-WW1 too, but for bureaucratic choices. Pre-WW2 the major reason for no major production of SLRs pre-1941 was lack of funding, desire to rearm as quickly as possible, which means focusing on existing designs, and bureaucratic hinderances. There were no technological reasons for the lack of WW2 SLRs in 1939, just choices and funding focus.
For this POD the big issue is simply getting the French Army to make a decision sooner about what it wants and be more accepting of the existing 6mm caliber instead of changing their minds and continuously upping the caliber demands, which created fatal delays in the program. Same thing with settling on the complex long recoil system for their SLR as well. Had they just gone the route they ended up with in 1940 with the MAS-40, the direct impingement recoil system, they could have had their SLR/autorifle weapon system ready in 6mm in 1905.
Umm... Most of your arguments are the same ones as I posited: Costs, politics and reliability. I agree that multiple semi-automatic rifle concepts were around prior to WW1. Its just that they were not yet suitable, for a variety of reasons, for front-line service. Even the Winchester Model 1905, as well as the '07 & '10 models, were not used in large numbers by any combatant in a front-line role. Like the handful of other pre-war semi-autos that saw service, they were almost exclusively used in naval service or air combat because there was no mud and/
or because they were sporting designs pressed into service. These were then superseded in that role once interrupter gear for machine guns were developed, relegating the semi-autos to rear echelon duties. Note that sporting designs are not always conducive to the demands of military service.
Every major power and many minor ones, Mexico being a prime example, knew that semi- or fully-automatic rifles were the future. The problem was not the availability of designs, it was that the technology,
specifically metallurgy, was not up to the challenge of delivering a reliable and affordable
military rifle until the late 1920s. All the small parts, springs, operating rods, cartridge cases, etc., keep failing under the stresses of
military use. This explains why machine guns, even light machine guns such as the Lewis & Madsen, were so much heavier than rifles. The extra mass of the parts could handle the stress and the design tolerances were not required to be as fine as those required for an automatic rifle. Issues with metallurgy crop up across all industries pre-WW1 with the Titanic being the most glaring example of this. The availability of reliable ammunition also restricted adoption of semi- and fully- automatic rifles in a military context, as most operating systems require ammunition of a consistent quality. Smokeless powders pre-WW1 were still relatively new and wartime production demands resulted in poor quality control for quite some time into the war. This issue still occurs from time to time, with the early M16s falling foul of this when troops were issued ammunition with a different powder load to what the rifle was designed for, and ARs use a direct impingement gas system, similar to the one you advocate for French adoption in 1905.
As for the automatic rifles that entered front-line service during the war, those had the benefit of war-time experience in trench warfare conditions and were driven by a need to increase firepower to break the deadlock. As such, compromises were accepted that would not have been pre-war, including those surrounding metallurgy. The Fusil Automatique Modèle 1917 and BAR are examples of these compromises, with the reliability of the FAM 1917 being a noted concern and the BAR weighing almost, if not quite, as much as a light machine gun. Note that even Britain had adopted the Farquhar-Hill Rifle by 1918 for general service, although the war ended before any significant numbers could be produced, despite the rifle being developed by 1911. Prior to this, it had only served as an aircraft weapon.
I also agree that by the 1930s it is politics and financial consideration that prevented the wider adoption of SLRs. This is because the tech had finally caught up with design. If this can be fixed prior to 1900, along with ammunition quality, then you could indeed have an SLR adopted by a major power earlier than OTL.