For us, 4 park days for the 4 of us would cost about $1480, so for two years that would be $2960. This is just regular base tickets, no park hopper, memory maker, etc.
Buying an annual pass for the 4 of us is $2356. So that is $604 every 2 years. So that would take 14 years to pay back $4500 on its own.
But for us, we do a lot of sit down dining, because our kids get worn out easily and it's a chance for them to rest/recharge so we can hit the park longer. Plus we like to do character meals and avoid the character meet and greet lines altogether. Last time we spent $1500 on food for the 4 of us, but that was crazy because we were trying to intentionally try a lot of character restaurants we hadn't before. So lets assume that's normally more like $1000, so $2000 for 2 trips. Take a 20 percent discount with TIW (or can redo number with 12% average discount without TIW), and subtract the cost of the TIW card, and that'll save us about $250 every two years.
So if you take those two together, that would pay itself back in about 10 years. Presumably the discount on souvenirs would cut that down a little further if you buy a lot of souvenirs (which we are guilty of).
Now, if you are the sort that would pay for park hopper and memory maker anyways (which we are not, but will enjoy having since it comes with the AP), that is an extra $470 for the 4 of us for one year, or $940 for two years. If you combined the 604 off base ticket price + 250 off food + 950 off for parkhopper and memory maker, that pays itself back in about 5 years, or a little sooner depending on if you also buy souvenirs. We would never pay the crazy prices for park hopper and memory maker at their full prices, but it does still add some definite value for us.
Plus we can be flexible about how many days we spend at the park vs resort, without having to predict the exact number of days in advance, which is a nice flexibility, and not that pressure to make sure you don't "waste" a ticket if everyone needs some rest.
That was enough we felt it was worth stretching - and I will admit some level of fear that disney is going to get worse with incentivizing direct purchases and raising the points needed for it over time, so we wanted to get in now so we can be grandfathered in going forward (hopefully, as they've grandfathered in the past). I'll be interested to see if they do something even more dramatic once Riviera opens.