I guess the reason why your client chose ExtJS of Sencha instead of Backbone/Ember/Angular is due to the level of support they can get from Sencha, not because of any advantages or what not.

Decisions like that don't come from developers, they come from the typical 'business guy'. Generally speaking bleeding-edge frameworks don't bode that well for any business guy. If a problem arises and a developer gets stuck, a developer just googles it and looks for ways to tinker with it - in the eyes of the 'business guy' they'd opt to throw money at the problem for it to go away, no time should spent troubleshooting or googling.

To sum it up: your client can't rely on a community of people that supports things only in their spare time. Thus by gaining comfort and trust from buying ExtJS contracts (https://www.sencha.com/store/extjs/) - whenever something goes wrong, your client's dev team gets stuck or needs training they can choke the hell out of Sencha for it.

Just let them go through with ExtJS. If the decision maker has consulted his devs long enough, it's a better decision. Being in one page and getting their things done quickly is far better than taking too much time learning things and contemplating on what framework to use.