So...I have something very similar...21 day I'm taking next month. We ended up buying the laundry package for the below reasons...that being said, I was still pissed af at the huge price increase.
1) We will need to do laundry in the middle of the cruise and not the end
2) They may not (as you mentioned) offer it onboard. However, we get 15% off packages as diamond so technically waiting we could get the same pre cruise price, but the stress (and the fact that MSC may not honor the diamond discount) made us decide against that.
3) The difference in price is only $36. It's more of a "principle" thing than an actual cost thing. When you step back and think objectively about it, it isn't that big of a deal.
4) We just got back from a HAL cruise where we paid $147 for unlimited laundry and were really happy with the price. Here, again, psychology comes into play. "Unlimited" for a, relatively, low price made us happy. But the reality is we only did about 60 or 70 items (and we were sending laundry about every other day for 2 weeks). Math-wise this is a *worse* deal than the 40 for $76, Now, we could split it up, use it whenever we wanted, etc., but overall cost-wise it was about the same.