You like free stuff, right? Who doesn't. There's a reason pretty much every top website is free to use. If Google charged users for every search, they would quickly lose the top spot. They wouldn't even have to charge much--fractions of a cent. Just requiring that you enter your credit card info would be enough to scare people off.
Every app (and business behind the app) has expenses. You have to pay for servers, salaries, and ping pong tables so your employees don't want to go home (ok, that one might just be tech companies). But these web2 business are still able to run profitable apps with free experiences.
In web3, your company is not the gatekeeper of your app. Customers interact with your dApps directly. If dApps want to compete with traditional web apps, free usage must be an option. On most blockchains today, gas fees kill this dream. You'd have to centralize to shield users from the fees. Koinos fixes this (no gas fees).
Free-to-use web apps are still monetized. Advertising, affiliate marketing, premium subscriptions, in-app purchases, etc. The majority of users don't pay to use the app, but if you didn't offer the free experience, you wouldn't get as many paying customers.
DApps need to be free-to-use by default so developers can leverage traditional monetization models.
P.S. You don't need to make money off of every single user.