Buy the outgoing model at a discount, or the new one?
When a new model launches, two machines are on sale at once: the new one at full price, and the outgoing one at a discount. The discount is not a gift. It is the market adjusting for the fact that the machine you are buying was, until last week, the current model.
The question is whether the discount compensates for the years you are giving up.
The arithmetic that decides it
Compare cost per year, not price.
Outgoing: (discounted price - its resale value after N years) / N New: (launch price - its resale value after N years) / N
The outgoing model starts cheaper and ends cheaper, so the numerator shrinks on both sides. What usually tips the comparison is the denominator: the outgoing machine has fewer years of useful life left before it stops receiving operating system updates, and that is a hard boundary rather than a feeling.
The boundary nobody prices
A laptop is usable while it runs software you need. Once it stops receiving major OS releases, it keeps working and stops improving, and some apps begin to require a newer version. That date is knowable in advance, and it is the same date for every machine of that generation.
So the real comparison is not "old model versus new model" but "years of supported use left" versus "discounted price". A 30 percent discount on a machine with two fewer supported years is not a 30 percent discount on the same product.
When the outgoing model wins
- The discount is large relative to the price gap between generations.
- You replace machines on a schedule anyway, so the last supported year is not a year you would use.
- The new model's changes do not affect your work.
When the new model wins
- You keep machines for a long time, so the extra supported years are years you will actually use.
- The new generation changes something structural: memory ceiling, port layout, thermal design.
- The outgoing model's discount is small, which happens when stock is thin.
The trap
Launch-week discounts are usually the smallest. Outgoing stock gets cheaper as it ages, but the supported years left also shrink, and the two effects partly cancel. If you are waiting for the bigger discount, price what you are giving up in supported years while you wait.
What to enter in the calculator
Use the discounted price for the outgoing model and the launch price for the new one, and use the same horizon for both. If the horizon you choose exceeds the outgoing model's remaining supported years, the comparison is fictional: you will not keep it that long, because it will stop getting updates before you are done.
A closing note
Neither machine is a mistake. The mistake is comparing sticker prices across two products with different remaining lives, which is the most common way this decision gets made badly.