The Ritz is known to give an fuel efficiency of at least 12 to 14kmpl, if your car is so low on efficiency you need to get it fully checked at the Authorized Maruti Suzuki service center and get the problem rectified.
Maruti has a good and vast service network and there will be no problem of maintenance and replacement of parts at later stage but the resale value of the car might decrease as the company has decided to discontinue the vehicle. Despite this, there will never be any shortage of parts as manufacturers make parts for next 10 years. Also, as Ritz share a good number of parts with its sibling Swift there will be no shortage whatsoever. If you would like to consider, the Indian auto market has many other offerings in the same segment such as Tata Bolt, Nissan Micra, Ford Figo, Honda Brio and Grand i10, Maruti Swift, Chevrolet Sail hatchback.