Is the Final Boss Veigar Figure already sold out? The arcade event has just started and I only read about it a moment ago. How did you calculate? It's is embarrassing and disappointing for the player - "here look at this extremely cool figure. You will never get one :)"
If the merch store comes with such negative experiences, you should not be surprised if some people don't feel like checking it again.
Make a pre-order system or something like that so everyone who wants a figure can get one, it's not that hard. But first come first serve makes more people unhappy than it makes happy. Or you could at least produce more than 5 next time so the stock lasts at least half throughout the event.