bitcoin hashrate does not have to exponentially increase.
difficulty can go up and down.
bitcoin rewards go down buy combined fee's can go up

in the future bitcoin mining will not be stuck to how many coins are produced, but how much fee's are accumulated per block

there are many variables that can influence a miners profits and influence how many miners are at work.
it does not require an ever increasing eternal rise of miners becoming central. it can actually cause distributing miners over more multiple locations

so your possible:
deadend 1: coin reward reduction
solution 1: more tx/fee's per block

EG imagine network was rewarding 0.78125(year 2032) but it was costing $781,250 to mine a block. meaning btc had to be $1m a coin to break even (if 1btc=$1m then 0.78125btc = $781,250)
but does not mean the price has to go up to over $1 a coin.. it can also mean more transactions per block to bump the reward to 1btc
say that 1BTC was $781,250, block reward was 0.78125($610,351.56) and combined fees were 0.21875($170,898.44) = 1btc($781,250) total.

deadend 2: physical electric limits
solution 2: distribute hash rate over more people in smaller devices in more locations
EG imagine instead of 10 pools in 10 countries. it was 100 pools in 150 countries
instead of slicing a pizza into 10 slices where they get to eat 14 times a day. its 100 slices meaning only eating 1.4 times day so each person has more they are wanting. thus keeping competition alive without overdoing power grids of 10 locations because its noe spread over 150 locations.
then next stage is that the difficulty doesnt need to then be soo high to need to cost say $15m a coin. it can actually bring difficulty down where each location doesnt need 10% of network power. and just needs 0.66% of network. meaning its more harder for any one location to 51%

deadend 3: price need to endlessly increase
solution 3: ever heard volatility. ups and down pumps and dumps, dips and hype. people can make money on the ups and downs, not just the ups

anyway. your worries and fears are not even your worries or fear, they are concerns which your great grandkids might have to consider which of the many variable paths to take next for bitcoin