There are distros targetting more specifically mainstream end-users, historically Mint and more recently Elementary (which does a fantastic albeit opinionated job with UI/UX, or so I hear).
The problem of general adoption however is not related to that. It's really not the best software that wins the wallets of consumers, but the best marketing — same for music, movies, vacation destinations, virtually any product: marketing 'bypasses' quality (it just needs to be 'good enough') by 'brute-forcing' awareness and familiarity (sheer repetition, seduction, etc). It's true there's a lasting power to great UX (ask Apple), but Linux desktop never even entered that phase for the mainstream (first you have to exist before we even ask if you're going to last).
Making the best Linux experience is thus a vain enterprise. It just won't sell more or less (if we agree that even free has a cost), it just won't bootstrap massive adoption. It's at best preparing the product for its encounter with the public, but it's not making said encounter more likely.
A strategy like Microsoft's with Windows shows where the battle of OS's is fought and won: with OEMs.
Microsoft correctly identified (just like Intel) in the 1990's that people would buy computers (thus OS, thus CPU) through middlemen, OEMs, the Dell's and HP's and ASUS's of the world. So that's who you need to sell on your product.
Apple is its own OEM, as they integrate vertically, and indeed the 'integration' between hard- and soft-wares is what sold most of us who ever bought into OS X or iOS et. al. The interesting take is that Apple for the longest time oscillates between 5 and 15% market share on average on all their products worldwide (much closer to 50% on star products in the US, nigh-zero in the poorest markets), and this is the nonetheless massive (9 figures) user base that guarantees adoption of their OS, which are designed entirely to sell the hardware by making it shine (what MS is now trying to do with Surface lead by Panos Panay, or Google with Pixels and other things).
This whole 'integration' space is where Linux never even tried itself, it seems, because there's never been a "Linux Marketing Department" nor a "Linux Sales Person" — when Dell does Linux, it's because a bunch of fans inside the company made it their project; when Red Hat sells Linux it's closer to the goal with OEMs but certainly not mainstream/consumer in that case. As for Ubuntu, I don't know who they sell to; they're certainly big on the servers, but absent of consumer laptops despite being totally usable these days (and most consumers certainly don't need the advanced features of paid O365, so that debate is moot in this case). There are Linux brands (notably System76 with their awesome PopOS), but they also don't target the mainstream it seems, or not in a 'big' way likely to turn market figures around.
The truth is that I don't think anyone with big-tech decision power really cares now that we've moved to IaaS and serving apps on containers (alternatively 'minified' through some CD pipeline from a common base) is going to increasingly become the 'easy' way to have apps 'just work'; and by that point who cares if it's Windows, Linux or even iOS in the kernel underneath. By 2030, I anticipate that OS/kernel won't make much more difference than the CPU you're using (tech people and nerds may care, most people just want apps X Y Z to run fine and they'll get just that). Whether server or client side, we'll all live on top of some common or at least expected abstraction and most of us will be none the wiser. I'm not sure BSD will be much around in this scenario, except legacy and of course as darwin for Apple.
Not saying I predict the future or this is what must or should happen, but it definitely can as I see it, it's a 'valid scenario' with a high degree of probability IMHO.