Currently, (mileage may vary in your jurisdiction) a party which gets 10% in elections at a particular level of government is entitled to (say) 10% of the available public funding. The party is resourced, not only for its electoral action, but as lobbyist or think tank, in function of its democratic legitimacy. This is why we have public financing, without which the left is completely kneecapped (absent mass movements of the working class).

Where does that leave parties in your sortition system? De-financed and de-legitimized.

Just because we don't like any of the parties much, it doesn't mean they don't have a useful function as mediators of the political system. Probably they need to be regulated more (transparency and democracy in their internal functioning would be a good start!)

_{It is rightly acknowledged that people of faith have no monopoly of virtue - Queen Elizabeth II}