"May is traditionally the weakest month for sales in Japan, so there's no good reason to react like you did."
What I see is that these last 3 weeks, last year the baseline could be maintained at 35000 with nearly nothing coming, this year, we have much more 3rd party games coming and yet it has collasped from 50000 to 26000 in 3 weeks, games like assassin creed and Sonic racing did not prevent at all this decline.
Before those last three weeks, Switch had built a year over year lead of ~300k units with hardly any noteworthy releases in 2019. There was NSMBU Deluxe in January, but not much in the months afterwards. Yoshi unsurprisingly sold far less than Kirby last year. In any case, Switch remains up year over year by a good margin and some weak weeks aren't going to undo that.
Using the spike of Golden Week as reference point for the beginning of a collapse is poor analysis (plus your 50,000 figure is the value from the year before). You might as well panic in January when sales are down after the holidays. Further bad analysis is when you mention games like Assassin's Creed and Sonic as notable highlights of third party support. A remaster of an IP that Japan doesn't particularly care about is a poor measurement and the same holds true for anything Sonic. Unlike in America and Europe, Sonic's popularity in Japan was always severely lacking. Even new mainline Sonic games struggle to pass 100k lifetime.