Problem is that your arguments will never carry as much weight as her “I’d always be worried about you” and “I can’t stand to live without you” argument around safety.
You can try the saving money argument, but that may come back to kick you in the teeth. My experience riding 10k per year on bike that gets double the mileage of my car and is completely paid for is that my gas savings covers my insurance, registration, self-done maintenance, tires and gear. I end up saving a few hundred a year, but I’d be TOTALLY upside down if I added a bike payment and full coverage insurance. Your mileage may vary, but I’d be careful about BSing the Boss.
I had to laugh about the “forgivenss” route. I tried that with a boat purchase several years ago. Wife came home to a boat in the driveway and gave up a “what the hell is that” comment. I guess when I told her I was going to go look at a boat she didn’t understand that the logical conclusion would be that we would all be looking at it at our home. Anyway, every day on that boat was crap cause she resented it. I finally told her that if she didn’t chill, I was going to sell it and start over. Within a year I sold it and then bought a larger one — which she had full disclosure and warning that I was buying. I never asked permission, but at least she was involved in the process — and now everyone is happy.
Good luck.