Not to say you're wrong, but it's worth noting that a 512k has more space on it than any removable disk system currently available for the TI, unless you count using an entire CF card. 180k is the largest reasonable maximum, since nearly everyone has at least DSSD. That's why you wouldn't be too likely to see a 512k cartridge that also loads from disk - the complexity of adding loading from disk to get just 30% more data, at a very slow comparable rate, wouldn't be worth it to me. If you went UberGROM cart you get 120k of GROM space which is almost as much space as the floppy offers, and it's almost as fast as the ROM for loading (at least when compared to a floppy disk).
As for filling the 1MB SAMS - again, where's it going to come from? Unless you are generating data or using a large map, I've done the math for that one already. If you're loading from 180k floppies you're going to be sitting there for a while as you swap six floppy disks to get the game started.
And 512k isn't the maximum cartridge size. We already have a 2MB cartridge laid out and in production, available today. Now you've doubled the AMS RAM size, you have plenty of room. If you were ambitious, your cartridge can hit 32MB without changing the bank switch concept - just keep adding latch bits.
In short... loading from disk doesn't give you anything you can't already get from cartridge. And the cartridge is self-contained, the user doesn't need to have additional pieces. The AMS can still be useful - RAM is always nice to have handy, but I don't see the value of a large cartridge combined with disk loading.