I definitely feel it's worthwhile to do the math. After all, most resorts have had a CAGR of 4-5% over the lifetime of the resort, and if salaries only raise 2% per year, it's worth figuring out if you will be comfortable with dues that increase faster than your salary increases over time. I think the vast majority of us do need, or at least want, to calculate future expenses, to at least ensure long-term that we still feel we're getting value from this, let alone making sure it will fit into future planned finances.
One of the things we did, which admittedly many on this board will argue against, is bought more points than we expect to use in a typical year. We did this for 3 reasons
(1) we want to be able to splurge from time to time on Cabins, or upgrade from Studios to 1 BRs and 2BRs
(2) we want to be able to treat friends and family some times to come with us to Disney, and thus want to be able to book Grand Villas (for example, we're treating a bunch of friends and family in 2022)
(3) on years we don't do (1) and (2), we want to be able to
rent out points to help offset the cost of dues. I completely get that this is not the wisest course, and that if you do the math, it is certainly not the most economical decision. I get that the wiser course may have been to take that extra money and invest it to pay for future, BUT, personally, we wanted the flexibility of having (1) and (2) when we wanted, while the option of curtailing back
DVC in the future, either by renting points to cover dues or selling some of our contracts. It's impossible to truly know what the future will bring, and while right now we believe we will always be able to cover our dues expensive with our salaries, I can't definitively say where I will be working and for how much in 10 years. As we could afford to buy more contracts than we need (and we paid "cash," using the advantages of the Disney Visa to spread payments over 9 months for 0% interest and get 2% back), this was the approach we decided to do after looking at the numbers. It is certainly not the right approach for everyone.