The payments coming due for contracted memory purchases are spread out over 5 to 10 years, not 1 to 2 years as implied by your "excess chips hitting the market in 2027-28".
For memory specifically, the counterparties are mainly "Google, Microsoft, Amazon, Meta, even Apple" like my sibling comment says (plus creditors most of us have never heard of making loans secured by data centers OpenAI and Anthropic plan to rely on).
Going public should make OpenAI and Anthropic a more attractive target for new large loans, again mostly secured by data centers.
If demand for AI services flatlines or if an international agreement stops the labs from training any new models (for safety reasons), then memory prices, GPU prices and inference prices will probably fall drastically, but OpenAI and Anthropic would probably survive that.