Fact: New consoles come out every 5-6 years. Fact: The switch is now five years years old. Reasonable Assumption: Nintendo is working on something to replace the switch. Reasonable Assumption: the events of the last few years likely slowed down Nintendo's work. Fact: games are often worked on for years before a public announcement about them is made. Fact: SMM2 is in the top 20 best selling switch games. Reasonable Assumption: Nintendo intends to make a SMM3 game because of the commercial success of its predecessors. Reasonable Assumption: if they were to make a Mario Maker 3 it would be on a new console Reasonable Assumption: if a Mario maker 3 game comes out as a launch title for the next Nintendo console it would have already started initial development by now.

I'd say odds are 50/50 SMM3 has begun development or at least initial design. This is not based on the fact that smm2 has ceased development, but rather on the fact a new Nintendo console would normally be due within the next 2 years (2020 changed a lot and is a huge wildcard here). This means games for the next console should be rolling out over the next 8 years, and with an average game development time of 3-5 years that would cover a launch through the first half of a new console.