Arena offers atleast six different formats to play. Making a deck for these formats can be radically different between Best of 1 and Best of 3 when taking into account a sideboard, sometimes requiring a different build of what is fundamentally the same deck taking two different storage spaces. Between four sets a year of new cards and countless content creators encouraging players to try weird and experimental brews, even a new player comes to a quick realisation:

A limit of 100 saved decks is WAY too small. I only play Standard and Brawl and even I am having find alternate ways to save and store decks. You already offer starter decks outside of the 100 deck cap so it's clearly not an issue of storage space.

Encourage your players to experiment and iterate by removing this inconvenience. 250 would be a good next step!