I'm starting to think that TSMC's stance that they're not going to increase production capacity may not have been as good an idea as they originally thought. They're leaving a crap-tonne of money on the table that disappears every time someone who was going to buy an AMD product ends up with something else due to a lack of choice in the matter.
GlobalFoundries abandoned 7nm process and doing that they made exactly same as you say TSMC did. GF was supposed to make many AMD products but they decided it (7nm process) was too expensive.
That's also one reason for chip shortage. TSMC 7nm capacity is around 140Kwafer/month and GF capacity for 7nm was supposed to be around 60-80Kwafer/month...
Also for TMSC, 5nm and other better ones are coming so there is no real reason to increase already old 7nm capacity any more.