the 3DS isn't even selling much worse than the Switch currently and could shock and end up at like 80 million at this point. I honestly think that the Switch ending up over the 3DS is extremely questionable, particularly with it being at a higher price point. could happen, but don't be surprised if it doesn't. Nintendo would need to produce a lot of systems and the 3DS had some great boosts from different models.
At this point I would hesitantly say that the Switch will outsell the 3DS for the simple fact that it's both a handheld and home console, but it's far from a given. A lot depends on if Nintendo keeps the 3DS line alive the next few years. If they were to, say, announce a big game for the 3DS next year then all bets are off. That system currently just seems like it won't die