We don't want to encourage anyone to stay on old browsers. All the exciting new features being added like gamepad support, fullscreen support, high-resolution timers, hardware-accelerated rendering, javascript engines fast enough to run physics simulations and particle effects, generational/concurrent garbage collectors for smoothness, Web Audio API, WebGL shaders, real-time audio and video streaming/peer-to-peer calls over the web (currently in development by browser makers), page visibility features (which also enables pausing the game when hidden), new form control types, Web Fonts, sandboxed iframes for secure game hosting, and other features which we don't currently use but might in future (mouse lock, features from CSS3 or the new DOM additions)... they're only in the IE9+, Firefox 4+ and Chrome generation of browsers.
Firefox and Chrome release updates every 6 weeks now, often adding more features too. The web is moving forwards fast. We're trying to keep Construct 2 up with the bleeding edge. Being at the forefront of technology is important enough to us that if an organisation does not want to install a free improved browser to support Construct 2, then we won't slow ourselves down by looking back.
By insisting we're also increasing pressure to these organisations to upgrade. Old, outdated browsers hold back the development of the web - just think of the complete nightmare that was IE6, and how it continued (or even continues!) to be a problem over ten years after its release! Note Firefox and Chrome automatically and silently upgrade to prevent this problem. So we want to strongly encourage people to use modern browsers, too. It's sort of "being cruel to be kind" in my eyes, because it is actually better for them too, even if they don't want it (these browsers are a lot more secure as well for example). And I am confident that despite possible short term gains, in the long term it is more beneficial for our business to insist on this: I wouldn't want anyone to still be using IE7 in 5 years time because we let them get away with using it...