I know there are a couple companies that will refurbish a pack for less than Tesla. I am wondering if there are any true aftermarket packs under development? Having an older Model S gives me anxiety...
Based on my personal need to pay Tesla 5 figures for a NEW (not reman) replacement pack over a year ago and closely watching the 2012-15 MS/X replacement pack landscape, I think there will be no NEW MS/X after market packs under development. Currently Tesla is the only one making NEW packs.
Everything else out there in the replacement pack world is 10+ year old material meant to last, at best, 3 years based on warranties. Further, due to small number of MS/Xs I doubt any entrepreneur can make a living researching, producing, and scaling NEW replacement MS/X packs. Due to volume of M3/Ys, there is a possibility of NEW replacement packs outside of Tesla.
Further, you are right that there are a couple companies that will refurbished a pack for less, the only one currently less is Recell on their 210+ pack replacement option but that is not an apples to apples comparion. If you look at Recell's 240+ option at $9995, then that is apples to apples comparison. Here's why:
$10k USD - That seems to be the current market rate for an 85 kWh used/reman/salvage replacement pack that gives the owner a rated range of around 240 miles. Just the pack. Any applicable labor, taxes, and warranty (explained below) are extra.
$10k includes Tesla, 3rd party professionals and salvage yards. Tesla bundles in a 4 year/50k mile warranty while others offer warranties at an extra cost. Each year of warranty seems to range from $1250-$1500/year. e.g. Tesla's basic $15k=$10k pack + (4) x $1250 warranty. Of course, Tesla does not break things down like this, but based on 3rd party warranty fees, a reasonable comparison.
So your decision around that magic $10k number for a 85 kWh used/reman/salvage replacement pack revolves around warranty and convenience of location/time.
P.S. - Add $4k for Tesla's NEW (not reman) pack.