Luigi's Mansion is a series with growth potential in Japan and its ambiguous release window has been narrowed down from 2019 to Q4 2019, so it's now supposed to release in proximity to Sword/Shield. Pokémon: Let's Go didn't do hot numbers in Japan, so LM3 can match that. Last year's Q4 also had Super Mario Party which sold steadily and it's doubtful that Mario & Sonic can match that, simply because the Sonic IP has hardly any pull in Japan.
In summary, I'd put it like this:
Super Smash Bros. Ultimate > Pokémon Sword/Shield
With LM3 in the same quarter, the lineup is more evenly matched and there's only a small advantage for 2018. Still, this was the scenario without price cut and revision, so a one year older Switch at the original price makes the hardware itself pose a challenge for sales. The more probable scenario is that there will be at least either a price cut or revision, and that makes a year over year comparison favorable for 2019 anyway, regardless of the launch month of LM3.
I know M&S at Olympic Games won't match Super Mario Party but don't you think it will preform better than previous entries in Japan considering the Olympics are being held in Japan? Heck didn't they have Mario jump out of a pipe at the end of last Olympics? So the legs might be decent on the M&S game especially if it promoted throughout next year up to the Olympics.