In my opinion, removing bolster completely is the way to go and can be done more simply than many are implying.
1. Make Infantry Sections a 5 man squad from the start.
2. Reduce the DPS of Infantry Section Rifles by 20%. Now a 5 man squad has the firepower of an old 4 man squad, but the durability of a 5 man squad. This nerfs the late game power of bolstered infantry sections but boosts early game durability slightly.
Additionally, if double BREN squads become too powerful with the above changes, then do step 3 and 4.
3. Limit BREN guns to 1 per squad.
4. Boost BREN gun stats to be more similar to LMG42, and boost cost up to 60 or 70 munitions
With 20% lower DPS, infantry section models won't be worth 28 MP per model. So you either:
1. Keep this reinforce cost and accept that brens will be essential, or
2. Lower their reinforce cost
If you go with 1, using unupgunned tommies is going to feel about as bad as using 4 man tommies now because each model isn't going to have the firepower of a 28 MP model. Now instead of making tommies feel bad to use until they get bolster, you make tommies feel bad to use until they get weapon upgrades. So then players will rush brens and have (relatively) early 5 man double bren squads because they'll be getting that instead of bolser. The performance loss is then really nominal since youre only dealing with 3 nerfed enfields, and you get 5 man, double bren tommies with a (5%?) slight performance reduction for 150/35 cheaper. As you noted, then you could limit the bren to 1 and make it similar to an lmg42. At this point though, they're basically just grens with extra steps (though maybe half a step stronger). Maybe that's fine, but it seems like it'll make the matchup up far more boring.
If you go with 2, then unupgunned tommies feel fine to use, but upgunned tommies become too efficient. So then you lower LMG performance, possibly limiting it to 1 as above but making it weaker than in the previous example. Then they're even closer to grens as above, which to me would be even more boring.
In either case, it seems very likely to me that you have to limit the brens to 1. Simple, right? But what about sappers then? I guess they also get limited to 1. But what about the vickers drop, does that stay super strong? With the resources you save from not having to bolster, you almost have enough resources to buy the halftrack too. That'll probably end up being too strong. So maybe you limit the vickers to 1, but how? I'm not sure that it's possible that one vickers take up two weapon slots, so you can try working around that - instead of limiting brens to 1, you just limit infantry sections (and sappers) to 1 weapon slot. Well now you have a halftrack that can do the equivalent of giving penals/riflemen/cons two mg42s... What do you do then? I genuinely don't know. And at this point, the suggestion doesn't seem as simple as we would have thought.
And that's not even the end of it. With bolster removed, do sappers start as 5 men too? Maybe they do...except then now maybe sappers become meta. 210 manpower for a squad that has a snare and can run up against grens and win, available fairly early into the game. Do you increase their manpower cost? 230? 250? When we get as high as that, we start to come up against the OKW issue with extra repair squads becoming more and more impractical. Is this acceptable, and if it isn't, is there any work around?
Now, imagine trying to pitch this simple solution to Relic. A simple idea that fixes everything and doesn't take more than 5 minutes before everyone is up to speed with the idea and is willing to accept it. Then suddenly someone on the team mentions any single one of the concerns I mentioned above. You now have to argue it down or come up with a work around. 30 minutes of back and forth and you come up with a work around like I did above, everyone accepts it, and Mirage is luckily around to confirm that the idea is possible (because the alternative is waiting ~2 hours for him to get back home to his computer and maybe shoot down the idea anyway, at which point the european members of the team would already asleep and conversation would have stopped for the day). But then the next day, someone realizes and mentions an issue that the previous work around causes. 2 hours of brainstorming, another day down, and we're lucky if the idea has even been fully thought through and all of the issues adressed. No one has even gotten to the numbers on these changes yet, by the way. Maybe another 3 days of discussion for that, and now we're a week in. Well, to Relic, this simple solution doesn't seem so simple anymore. Best case, you end up pulling back and debating on changes for another week before you make it to a point where you're shipping half of the idea that you came up with. Worst case the idea gets shut down entirely. That's how simple solutions tend to work out.
If it sounds like I'm ranting, it's because it always seems like a lot of people are very naive when suggesting changes and I don't think they realize how much time ideas take to make it through. They suggest a simple, elegant solution to a new patch issue that was "so obvious and everyone should have seen coming" (shockingly, most people usually don't see these issues ahead of time for some reason) and then go about their day because they don't have to address the 5 other issues their simple solution causes.