Apparently if I waited 13 days to buy Premiere Elements 15, I would have a free upgrade to 18. But no. Adobe chat representative denied me and said it's "company policy". Thanks for making a new customer an angry one.
If you had tried a different support agent or asked for a supervisor at that time, you may have been able to get the free upgrade, but complaining about it now, unfortunately, will not likely result in a free upgrade.