I have a pretty slow machine and GNOME Web is unusable on it. The performance is not comparable to Firefox or Chromium. Extensions are very important for regular people apparently (judging from old Firefox Play Store reviews). Also Firefox and its derivatives are known for advanced privacy features that GNOME Web doesn't have and likely will never have because GNOME is about extreme simplicity by all costs.
I like GNOME Web and I really can see myself installing it on an old person's computer because of awesomely simple UI. But it's not for most people and I'm afraid that without commercial support there won't be any good regular browsers based on WebKit. I wish engine-specific features didn't exist. Everything would be so simple without them.