Lets say that you play this game for 1 hour every day and you complete all the daily missions and weekly missions as soon as possible. For the daily missions you get 40 acoins every day. For the weekly you get 200 acoins every 4 days.

That means you would get 1,200 acoins for a month from daily rewards and 1,500 acoins for the weekly rewards. Together that is 2700 acoins.

Lets say that you somehow win the vault once per month. That would be 3,425 acoins.

Adding the acoins you get from crates one hour a day would get you 20-30 acoins a day. That would be 4175 acoins monthly.

Since you get around 4175 acoins monthly that means you would have to grind for an hour every day for two months for one legendary mech or weapon. 

Two months is a lot! Especially if you want to get multiple legendary weapons.

For you to get every weapon in the game it would be 250,000 acoins which is 60 months to get all of those. That's not including upgrading.

FIVE YEARS!

That is why mech arena needs more mechs and weapons worth credits.