Well since it seems so clear that the Bren was they way of the future why did most of the major armies of the world go in a different direction? The Americans went with belt feed MG's, and auto select rifles. The French went with belt feed MG's inspired by the MG-42. The Germans who faced the Bren stayed with MG-42 derived MG's. The Russians did their own thing, with belt feed MG's. The Italians the same. India had tons of Bren's, but they bought FN, Soviet, and Israeli belt feed MG's. Israel actually bought FN-BAR Model D's. The UK went over to the L7 Series MG's, to replace both their Vickers, and Bren's.
It was the wave of the future in the 1920-30s, no one claimed it was post-WW2. Though it was still a very good LMG into the 1990s.
As to US Army post-WW2 small arms decisions....they weren't known to make that best ones. Another poster already covered your claims about other nation's post-war MG decisions, so I will leave those claims alone and defer to that poster.
It seems the major armies of the world followed a German Model, and provided their men with belt feed SAW's. If WWII infantry combat showed so clearly that a top loading, magazine feed LMG at the Squad Level proved superior to GPMG's, why didn't they follow up with SAW's based on the Bren?
Right, most armies aped the Germans, but ignored where they were headed in terms of equipment organization and even rejected what the Mauser designers had developed post-war that was an evolution of where the Wehrmacht was headed in WW2. As US combat experience showed the belt fed, crew served MG at the squad level was on the way out. They tried to make it work with the M249 as a single man system in the 1970s, but that still ended up very heavy and in some services replaced by automatic rifles with box magazines, which is what the Germans were headed too at the end of WW2. It all comes around...
Ultimately why no one really followed up with the box fed magazines LMG for the squad after WW2 is because those that used them in WW2 just kept the Bren into the 1990s and in some cases beyond. Hard to improve on perfection. Meanwhile NATO was largely at the mercy of whatever the US wanted for equipment commonality, so if the US picked wrong then everyone was kind of dragged along. Once the Euros got beyond their WW2 combat experience they forgot what worked in a peer level war and got more and more into the colonial small wars mindset. Prior to that though generally what the Euros wanted was different than the US (after look at the EM-2 saga, CETME, even some of the German designs). The one exception of course was a non-US Army American designer who came up with a weapon system the Navy SEALs loved:
American Joe's didn't want more automatic weapons because of the poor performance of the BAR. In any firefight soldiers want to put out as much suppressive fire as possible. British Tommy's didn't say, "We don't need any Bloody SMG's, or Semi-Auto weapons, we got a Bren with us." German Landser's didn't say, "We don't need any MP-40's, or MP-44's, we have an MG-42 in the squad." The Red Army had whole companies, and battalions armed with PPsh-41 SMG's. The Germans used all the captured Russian SMG's they could get their hands on. The British took a lot of Thompsons, without complaint, and the SAS used Browning MG for their fames deep penetration raids in North Africa. The British mounted Browning's on many other vehicles including Sherman tanks. They couldn't have been that bad of a weapon if the British manufactured it, and used it so extensively.
The GIs wanted what the Germans had...having not really used it in combat and not understanding the difficulties in using it.
Tommy's didn't say that, command told them what they were going to have and to like it. As to the SAS's use of belt feds, that was probably more a function of what they could get their hands on and the US was sending them all the Thompsons and Brownings they could, so that was what was used. BTW the Shermans came with Brownings.
Frankly though I don't know why the British didn't just adopt the Browning over the BESA given the weight and complexity of the BESA. Granted though the lack of a QC barrel was a pretty big hinderence.
Same thing with the Landsers, it was largely there is what we've got, use it. And then they supplemented with captured automatics, especially the PPSH 41, which in some ways arguably created the pressure to adopt the STG.
The Soviets adopted SMG companies because it was much cheaper and easier to make and use than bolt action rifles and because they saw how well the Finns used them, so they tried it out. Of course they subsequently added LMGs and rifles back in to SMG companies when they got out of Stalingrad, because in open terrain they needed longer range support. Not only that, but they developed a belt fed SMG and decided against adopting it in favor of their own intermediate cartridge designs.
I think you overselling the Bren, when you say it out preformed M1919A4, and matched the MG-42. It was a good weapon for it's day, but it wasn't the best MG of WWII. That most of the worlds future MG's followed the MG-42, rather then the Bren is proof of that. All the armies of the world aren't stupid, or unable to analyze combat data.
You're allowed your opinions. The QC barrel though does give it a sustained fire edge over the M1919 though. In terms of the MG42 vs. Bren that debate is endless, it's just that IMHO as a purpose designed LMG it's all around virtues give it the edge on the GPMG in light pattern operating as an LMG. The GPMG is a jack of all trades, master of none type weapon, which is why it has been largely kicked out of the role of LMG in favor of the SAW and is mostly used in MMG roles today. Even the HMG role has been taken over by the M2 .50 cal or the newer LWMMG .338 cal.
Best MG and best LMG are different things. Without a doubt the MG42 is the best GPMG of the war, but in the LMG role it wasn't best suited for that. It effectively was a MMG shoehorned into the role. That certainly gave it a firepower edge on the defensive, but offensively it was just too heavy and cumbersome to operate effectively which is why it got replaced by the STG at the squad level by the end.
No surprise that most post-war GPMGs were copying the best GPMG of WW2, but again that was a specific role, we're talking about LMGs here.
The US Army Ordnance bureau from 1945-1970s would challenge your claim that they weren't stupid or able to analyze WW2 data.