That is neglecting network effects. Less than 10% of the US population is vegetarian, but if a restaurant doesn't have any vegetarian options they lose business not just from that 10% but from any party that has a single vegetarian. Likewise, if a website has any social network effect, disregarding a portion of the population will decrease use from a much larger percentage than those directly affected.

Furthermore, even if your site functionality has no social networking component itself, all business are subject to the network effects of word of mouth. People are much more likely to share negative experiences that positive ones, so if 1/50 of people find your site to be broken, then a considerable amount of feedback online will be negative and will harm your reputation for the entire market, not just that 2%.

Finally, in business you have to work hard to win over even a small portion of your total addressable market. Artificially decreasing your TAM can be fine if it is an intentional strategic decision to focus on a specific market, but pointless to exclude people without good reason. Not having vegetarian options at BBQ restaurant in Texas is harmless - no one goes there for that, but if you are running a more general restaurant it would be foolish not to have a few vegetarian options. Excluding people because your web developers are too lazy to use approaches that have worked fine for the last 20 years and need to use the new shiny is even more foolish.