There's no debating that milk is heavy to digest, more so than water infused with whatever you want (tea, rice, soy, oat, almond...), whatever your ancestry. You actually need some enzyme to cut it down so that you can metabolize it. This process doesn't exist for infused water.
> The solution then is to just not drink milk, not substitute it with oat-canola slurry.
No, you can buy very fine "vegetable milks", just not oatly. I don't have any experience in US but in every west-european supermarket you can find heaps of them in the organic section. And if you wanna argue about the price: just make it yourself, it's dirt-cheap, just buy bulk cereals/beans (oat, soy, rice, chickpea ... be creative it works with mostly anything), boil it in water, mix it and drain it. Ta-Da, you just got pure vegetable milk for pennies and as bonus heaps of dry draining residue you can use as wheat replacement in cakes or croquettes (or used as semolina).
Oatly or cows milk can be part of a perfectly healthy diet provided use in moderation. If you’re having 3, 12+oz lattes maybe you should consider not doing that.
My favourite in the uk is "innocent".
But I think if you're going to get wound up about canola oil, there are bigger targets than milk substitutes.
You can of course say you don't want to (or can't) drink milk for a very long list of absolutely fine reasons. The solution then is to just not drink milk, not substitute it with oat-canola slurry. Or you could reduce your consumption to the point where its price can go up (as more of a luxury product) and allow manufacturers to treat animals ethically. Or buy it from responsible farms.
You can also make the point here that there is a great number of people who, when faced with the choice of "drink less milk" vs "continue cow abuse", will pick the latter because they like drinking milk more than they care about cows (which are bred specifically for human consumption). In that case you could argue that substituting Oatly for milk will make the world better, because it lets them lower their milk consumption without requiring sacrifice on their part. The thing that worries me here are second order effects - if Oatly is indeed not very healthy (which it sounds like it isn't, at least compared to regular milk), then these people will then suffer lowered cognition and worse health, leading to them making less than ideal choices in democratic elections, and putting more strain on the health system.
Again - there are good critiques that can be leveraged against our current farming practices regarding milk production and more, but I don't see the solution being switching to synthetic alternatives with potentially harmful effects that marketing makes their best to prevent you from knowing. Just go without and advocate for change.