>> I fear bike manufacturers are switching to less repairable materials and proprietary parts to increase profits.
So I kinda disagree, there is greater interoperability in bikes now than there ever was(imho). Many SRAM parts can be used with Shimano parts and vice versa, nearly everything is still easily disassemblable and repairable by someone with a work stand and a set of tools.
The biggest "threat" to bike repairability is unfortunately electrification - motors and batteries are essentially black boxes that while repairable(nearly all ebike motors are right now) the skill level to do so is significantly higher. And batteries are of course disposable, no one repairs them as far as I know. Which is a shame because buying an ebike has been completely transformative for me with MTB as a hobby - it removed some mental barriers for going out on my bike out of fear I'll get too tired to continue, with an ebike I go much more often and ride for longer and further. But while I can do 90% of the maintenance on it myself(and I think anyone can with a bit of spare time), if the motor goes I'll have to rely on a third party to fix it.
Alloy, titanium and carbon fiber are lighter and that's what most consumers want, having watched Tour de France and bicycle industry marketing materials. Big brands facing increased competition from Asia started to put carbon frames of questionable quality on the market, so if you want an okay bike you are best served by alloy, because they're mass produced and the process is quite optimized and well understood. Titanium is basically unrepairable without special tooling and know how and it feels basically the same as alloy. It's stronger and more corrosion resistant.
The bike industry also puts a lot of new or proprietary standards on the market, especially when it comes to bottom backets. Other new standards include different fork spacing, head tube diameters, rear suspension couplers, proprietary frame integrated suspension and more recently integrated e-bike batteries. The worst offensers are Cannondale, Specialized, Trek and Cervelo.
If you want a bike with sensible standards you want a BSA threaded bottom bracket if it's a metal frame or BB86/92 press fit if it's a carbon frame, a fork with a 1 1/8" steerer or tappered from 1 1/8" to 1 1/2" steerer if it's a mountain bike or a high end road bike, a 34 mm or a 44 mm head tube, 135 mm O.L.D. rear and 100 mm front fork spacing for quick release skewers or 148 mm/110 mm boost spacing if it's a newer mountain bike or 142 mm/100 mm spacing if it's a newer road bike with thru axles, a seat post diameter of 27.2 if it's a road bike or 31.6 mm if it's a newer mountian bike. For brake mounts I prefer IS brake mounts because they're robust, don't have threads to cross thread and misalignment issues are not common, but modern road bikes come with the flat mount (FM) standard and newer alloy MTBs come with post mount (PM). What you probably don't want is a mountain bike with flat mount brakes, funny axle standards, direct derailleur mounts, integrated seat posts, integrated cockpits, full internal routing, lefty forks, integrated frame suspension and so on.
Is the frame really an issue though? I don't aluminum frames break that often, I'd bet that's probably one of the least common problem for modern bikes (gears, brakes, suspension etc. are way more fragile). Most bikes are likely to be discarded way before the frame would need to be repaired. It seems totally like a red herring to me..
> Steel framed bicycles are good enough for the vast majority
Well most people don't seem to agree. There are still plenty of steel bikes available, even relatively high-end ones, they are just not that popular.
While horrifying for your friend, it's very low on the list of the risks that come with bikes.
I ride road/street almost exclusively, and the Bay Area roads don't do my bike any favors
Steel framed bicycles are good enough for the vast majority. I fear bike manufacturers are switching to less repairable materials and proprietary parts to increase profits.