Miners not getting paid enough rewards in transaction fees so either the price of bitcoin must double constantly or the hashrate crashes massively as miners quit and makes attacking nodes feasible.
The hashrate is an irrelevant metric as rightly said in the first tweet. What matters is the cost to attack, which is directly related to the reward in terms of purchasing power.
2. Bitcoin L1 gets mass adopted either through using the UTXOs themselves or by using some project on L1 like Ordinals, Runes. Fees skyrocket to 100+ sats/vbyte, and regular people use Lightning instead.
Those who will broadcast the majority of on-chain transactions in the future will either be large holders or L2 providers. For example, Ark servers, Lightning server providers, exchanges. Multiple transactions can be batched in one on-chain, and therefore ordinary users will just pay a fraction of the cost.