It’s a pretty good argument really. Bags are useful, as are bikes. Different types of bags can be useful in certain circumstances, as can different type of bikes. People tend to switch bikes/ bags for fashion reasons as well as practical.
It’s also possible to get away with “only” 1 or 2 bikes/ bags, but you may be compromising suitability slightly by doing so. You can ride a mountain bike for “gravel” riding, you don’t need a gravel bike, but it won’t be as much fun. You don’t want a gravel bike for utility, you want it for enjoyment. Biking is only useful if you use it as a means of transport, not riding for riding sake, and any bike can be used for this.
As a society we love buying stuff that we don’t really need but enjoy. As long as you can afford it then buy whatever you want to, whether bikes or bags.