Yeah as above.. I wouldn't even think about changing the gear ratio unless it's all knackered any way. 11 speed cassettes are a lot more expensive that 8 or 9 speed cassettes, and they are a consumable item they will wear out.
And if you go single speed on the front ring, you might have chain line issues to consider.
And if you go single speed on the front ring, you might have chain line issues to consider.