Well they already started migrated to Switch, but in bigger capacity they'll started migrating Switch has lower price point or Switch Mini/Pocket revision, or when Animal Crossing or Pokemon are released.

1) Price drop or a cheaper version
2) Big handheld franchises like Pokémon and Animal Crossing.

I could definitely imagine something like this:  a Switch "Mini" of sorts.  It would operate the same as a regular Switch but with no detachable Joy-Cons, however the system itself would be able to be paired as a controller for any other complimentary system.  This way you can still get the same "play together" experience you see in the commercials while also having a more ergonomic and personalized SKU, even if you don't have a Joy-Con to spare.

As a WiiU and 3DS owner, the progression to Switch was very natural. I got one at launch and havent looked back.
Still waiting for some of my favourite 3DS/Handheld franchises to come over, especially Ace Attorney and Proffessor Layton. But the fantastic Indie releases really have picked up the slack in that regard.

Big waves will come with Animal Crossing and Pokemon obviously, but there's still people entering the 3DS ecosystem only now and it will take those people a pricecut and possibly a hardware revision to come over.

I imagine early 3DS adopters have already started to transition over to Switch. It'll accelerate in the next 12 months, especially if Nintendo manage to get Animal Crossing and Pokemon out in that time.

Its already been happening since launch. I don't understand why one would think it isn't happening already.

This has been happening since launch for most of the people I know including myself. I've put more hours into handheld/portable consoles than placed into my PS4/PC.

I've had many 3DS's for various reasons...and got a Switch a week prior to launch thanks to Best Buy (no games though due to separate shipping...). I still play both even today. The Switch just took any remaining life that was the WiiU and a part of the 3DS.

The few holdouts I know that are still with just the 3DS is mostly due to a main Pokemon series not releasing on the Switch yet. While a few others won't get a Switch until a Mini version comes out... I think a large contingent of those types of gamers will migrate over once their favorite series releases.

Exclusive Pokemon and Monster Hunter... I suspect both, especially the latter, would make the Switch damn hard to find for months.

The 2 things you are asking for both eliminate the Switch's main feature and selling point:  Portability.  If a Switch TV can only hook up to the TV, it's not a "Switch" anymore.  The Switch concept is being able to seemlessly take your console gaming session on the go with you.  I don't see a version being released that can't do that.  Also, why ditch the joycons?  Playing with detached joycons is the most relaxing way to play games since the wiimote and nunchuck.  I do connect them to the joycon grip in certain situations, but the majority of the time I layback on my couch or recliner with each joycon separate in each hand.  Why eliminate the option?  As for the Switch XL, a larger capacity battery or screen would be nice, but the Switch screen is already quite large as far as handheld devices go.  Something larger would hinder it's portability, which again would counter its greatest asset.  In it's current state, I can slip it into most of my jacket pockets without even detaching the joycons.  If it can't fit in my pocket, then I can't take it with me through the park while I walk my dog.  For me, that eliminates at least an hour of time per day that I've gotten use out of the Switch, therefore reducing the device's usefullness.

I think the transition has already begun. It will take a couple more years before everything is finalized.