Many New Cars Have No Spare Tires

If Mfgrs weren't worried about costs there would be a lot less recalls. GM ign switches mopar mini vans with tailgate recall etc. They may pass on the cost but never make the upgrade. $1.00 saved on 500,000 cars is 1/2 million.
Recalls are the result of poor engineering. It doesn't cost more to design and produce a good part. Ignition switches that can withstand they weight of keys and fobs is a good example. Every mfgr always did that since day one. For GM to design always the harsh detents in their switch was just ignorance, plain and simple,,, and it cost them in the long run. I see the lack of common sense here, there, everywhere.
Some of your points I can't counterpoint. They do need to increase interior space for todays fatter population, along with fatter panels to contain air bags, in todays smaller vehicles.
Not only do fuel millage mandates play a part but 5 tires to dispose of or recycle with every 4 tire vehicle ads up too. They still haven't found a economical way to recycle a tire.
What we end up with here is a top 10 list, "Why to delete a spare tire". Cheers