You know, I'm genuinely not seeing that ?
The only thing I can attribute that to is that a) I'm not dialled into the noise as some people here or b) that I have a "false" floor under the main boot floor that sits on top of the spare wheel / Sub-woofer with a heavy carpet insert in it (to act as a shelf for storing items under the main floor panel)? But overall, I don't find road noise, wind noise etc at all objectionable on the GTI (and never did on the GTD's either, although there were a little louder). I certainly for 100% don't have a myriad of rattle or squeaks in the car, from that perspective it's commendably quiet.
Also, don't always believe the grass is greener on the other side as BMW are far from perfect either. Whilst I would generally agree that the interior fit and finish of the 1 series is better than that of the MK7.5, it's not leaps and bounds (at least that's my personal impression) and indeed I thought some of the lower down plastics were actually of a worse quality than VW. Where the 1 series scores over the Golf is that it's a brand new platform with the latest hardware and software, whereas the Golf 7 is essentially an 8 year old design.
I've been over on the 1 series forums and there are for sure a lot of ex GTI and R owners making the switch across to the M series, but hand on heart, is that really because it's perceived as a better quality product, and mainly the fact that owners that were perhaps looking for a natural upgrade to the MK8 GTI and R, are just not feeling the MK8 Golf and are somewhat disappointed and underwhelmed with what they see ? I think the M135i is perhaps what MK7.5 and 7 owners where looking for in a MK8 Golf maybe ?I've also been on the 3 series forum as well (another consideration for me), and whilst it's obviously a different class of car to a Golf, they are not without their own problems, and it seems BMW's online software updates can easily brick your car (and have done to a few members).
One thing I am pretty certain about is that when it comes time to part with my GTI (whenever that is), i'm 90% sure that a MK8 won't be a consideration for me as I just don't like what I see. I'm also not feeling the Mercedes Benz A class (have loads of them at work), so by the process of elimination, BMW sort of rises above the other choices, as for me, it's the most Golf like (I know, a compliment and an insult all in one).
For me, the 1 series caters very well for existing and perspective R owners, as they are very similar cars in many ways (i.e size, AWD, power, performance etc), but but us GTI owners, there's a gaping hole in the engine line up with the 1 series. You either have a choice in petrol of a 3 cylinder 140hp model (118i) or the full fat M135i (at nearly £38k before options) and nothing in-between. There's definitely a market I think for a FWD 120i with a de-tuned M135i engine producing 220-250ps for example. Add on metallic and adaptive cruise control pack and you are precariously close to £40k and the dreaded luxury tax bracket.
The second issue for me as a private buyer is depreciation. I've looked at a March 2020 M135i with options to a list of £43.5k with 5500 miles, being sold for £31,500 (before negotiations) that's nearly a 28% devaluation in 5 months. Similarly I've seen a beautiful 320i M-Sport with all the toys (list £44,800), going for £30,750 - that's over 31% depreciation in just 7 months (Feb 2020 car). So whilst I do think a BMW will most certainly be in my future as my next car (when ever that is), it won't be new but a few months old. Sure I might not get the exact spec I'd ideally want, but you save a bucket load of money. Company car drivers obviously have different considerations and most likely wouldn't be considering a M135i anyway but probably a 330e ?
One thing is pretty much for certain though, I think BMW and Merc, will be picking up a lot of hot Golf owners over the next year or so. Of course these are purely my views and expect everyone else to disagree with me 
I (also) readily accept that my perception is just that, mine, and that others may feel differently... I had not really put my finger on much of what I have been saying about the GTI until I drove the BMW then got back into mine and I started to notice how differently (let's say) the VW was in allowing certain noise back into the car... To be honest, I have been more preoccupied with the individual rattles and stuff that I had not really noticed the more general thing...
Also, I do not dislike the GTI and I am genuinely not trying to bash it... It is a very popular, very successful car and tons of folk are extremely happy with their car...
Your point about the engine gap in the BMW line up is well made... In the previous model, there was a 125i that was precisely the spot you were looking to fill and, yes, like GTI vrs R, it sat about £5k under the (then) 140i... What worked against its sales was that the 140 was mostly attracting such huge discounts that you could generally buy the 140 cheaper than the 125i...
As for depreciation... I would be lucky to get £22k for my car now... a car that was listed about £37,500 15 months ago... So I think that initial hit is common to many...
