Its a transition period, 3DS still sells decently and has a large active install base and likely generates a good deal of profits for Nintendo so it makes sense to continue to give moderate support to the system unlike Wii U which sold like shit and lost money. Think of it like GBA & DS. Nintendo said DS was not a replacement for GBA and that was true for about 1-1.5 years when they both sold well and recieved solid support. Once DS got a price cut, a revision and some big hitters we saw it replace GBA. That is what is likely to happen for 3DS & Switch, 2017 and early 2018 will likely see both devices with mixed support but by mid-late 2018 3DS sales and support will likely have dried up in favor of Switch.
I can't agree with this part. GBA and DS really ran side by side, so that Nintendo could keep both options alive. The GBA even got better first party games during 2005. That's not going to be the case for 3DS vs. Switch at all, because Nintendo's new console will absolutely destroy the 3DS this year in terms of first party content. This transitional period is like DS to 3DS and other previous handheld generation transitions, except for GBA to DS. Nintendo's old handheld has always got additional first party games after the new one was already out.
People in this thread interpret too much into the statement that Nintendo will show content for the 3DS at E3. Firstly, about the only new announcement anyone realistically expects at this point is Pokémon Stars as a 3DS and (likely) Switch game. Secondly, Nintendo's unreleased games (Pikmin, Ever Oasis) obviously qualify as content. Thirdly, third parties are not done yet with the 3DS and their games qualify as content too. Reggie didn't say anything about Nintendo announcing new 3DS games.