Remember last Christmas, when publisher QubicGames gave away 10 Switch titles for free? The kindly promotion – which meant you could claim 100% off the price of 10 of the publisher's titles, as long as you already owned another QubicGames release – propelled the free games to the top of the eShop charts, because Nintendo ranks downloads over 14 days, regardless of how much a game costs.
However, we won't be seeing this kind of promotion again, as it has transpired that Nintendo has closed the loophole which allows publishers to give away their games for free as long as the buyer already owns another one of their titles and still have them appear in the charts. The promotions can still be done, but the publishers won't benefit from an increased presence on the eShop – which basically makes such a promotion redundant.
While this clever way of "gaming" the eShop is no longer a valid option, the issue of companies applying deep discounts to their titles to boost them to the top of the eShop charts remains, and is unlikely to vanish as quickly. Games can still have their prices slashed by 90% and still be included in the charts.