However, we always have to take into account that even if such signatures are introduced (that would not need a hard fork), it's not necessary at all to make them mandatory, until a specific point in time: when quantum computers are so fast that they can apply Shor's algorithm to a public key in 10 minutes. Until then, quantum signatures can be purely optional.
As mentioned delaying the congestion doesn't change the later blockspace nuke outcome while custodian, CEX and Treasury will all upgrade asap anyway for risk management.
That needs and incredibly powerful quantum computer. If we see Shor's algorithm in action, then we'll first see "hacks" where old coins were stolen. But even if Bitcoin devs wanted to prevent that, they can simply ban P2PK and similar techniques.
Bitcoin compete only with other chains becoming quantum safe and better store of value first. It will loose relevance and face liquidity erosion if not Hardforking Blocksize to upgrade as no consensus will be found on disruptive softfork (signature introduction event)
And even if that happens, Lightning, Ark, the tBTC (Threshold Network) sidechain mechanism and other second layers will ensure enough capacity for a blockspace hardfork being unnecessary.
Because of the high congestion resulting from the softfork all those will ultimately force high centralization within shared lighting channels under potential censorship of centralized operators while having no real self custody.
If mainchain is not accessible to all anymore that only means sovereignty is lost for most but Institutions.
Softfork just doesn't look very great really