Uhhh, you do realize it's possible to sail using a headsail and trysail, or even under a jib alone, right? Hell, many folks sail most of the way across the Atlantic Ocean under headsails alone, after all...

Plus, what's so appalling about the prospect of having to rely upon an engine in a pinch? The overwhelming percentage of professional mariners worldwide do so 24/7

I'd bet if you tallied the number of boats in the ARC, for example, that are carrying spare mains, the percentage might easily be under 10% of the fleet, and largely confined to the largest of yachts...
Sorry, but I think you're simply not appreciating the way most people use their boats... Assume a family of 4 is making the same trip as you, in the same boat. Do you seriously think they're gonna be stuffing the vee berth full of spare sails?
I'm presently packing my boat for my trip this summer. She has considerable interior volume for a 30-footer, and I've configured her with extended cruising in mind, building in oversized water and fuel tankage, and the effort to maximize and make 'creative' use of storage space. The sails I'll have aboard are a main, 130% genoa & staysail that both live above deck on furlers. and a trysail bagged at the base of the mast when heading offshore. Otherwise, it gets stowed below with a storm jib, spinnaker, and a Code 0...
Never ceases to amaze, how much crap I wind up stuffing into that boat for a cruise, and how quickly much of it occupies the limited available space. After wedging my folding Northill into the big locker beneath the forward berth, then laying my folding bicycle on top, a lot of that space is
gone... Sure, I could stuff a spare main in there instead. But considering I have yet to EVER have the need of a spare mainsail, I'll take my storm anchor, and bicycle along instead...
I'll be headed north, so I'm dragging along a lot of cold-weather gear. Multiple sets of foul weather gear, survival suit, series drogue and para-anchor, 800' of shore lines, liferaft, a large ditch bag, Honda 1000 generator, sufficient oil for 2 complete oil changes, tools and spares coming out the wazoo, a 6-gallon jerry can for fuel, another for water, an outboard motor, 5 large fenders, a massive pile of paper charts and enough books to get me thru the summer, the list of items of considerable bulk just goes on and on... Since I'm obsessive about keeping my decks clear, ALL of that crap goes in the lazzarette/cockpit locker, or the quarter berth/garage below... Would I swap out any of those items to make room for a spare mainsail? Hmmm, I don't think so...
Obviously, your choices work well for you, for the sort of sailing you're doing at the moment... But, I think if you do find yourself starting to really go places with that boat, and needing to carry the sort of gear, stores and provisions required to be self-sufficient for longer periods of time, you'll realize there simply isn't room for all those spare sails you're carrying today...