Disclaimer : apologies if /presumptuous/out of place/well considered already/naive/plain stupid/. Just finding out about bitcoin.

According to https://blockchain.info/charts/hash-rate?timespan=2years&daysAverageString=7 currently nearing worldwide rate of 6 * 10^18 hashes/s but growth has slowed over the last fortnight to nearly flat (Bitcoin cash effect).

I thought mining would be fun to try - Antminer S9 costs $2500 plus 1kW or so electricity (=$0.05*24*365=$500/year) for 12 *10^12 hashes/s. So would mine a block every 5 * 10^5 blocks or 10 years.  A 2 year lifetime with a current reward of 12.5 BTC or $40000 or so, plus say average of $10000 fees gives expected reward of $10000 compared with $3500 costs.

(Incidentally what is going on when  that Antpool mines blocks with no transaction fees in it i.e 481053).

So just profitable at the moment but BTC reward will reduce by half in 3 years time and BTC value fluctuates.

I guess that is a free market law : if very profitable then more will do it, if not profitable then will stop.

My question is : What happens when world hash power declines because not profitable to mine?

It seems to me that Bitcoin security is based on ever increasing hash power. If the value of BItcoin halved and the BTC reward halved then  the impetus to develop more hashrate  and faster machines would disappear and the world hashrate would decline. If a mothballed mining factory could muster more power than the future world hashrate wouldn't that undermine bitcoin security. I can't see that mining fees and BTC value growth are guaranteed  to cure the problem. Is BTC is a victim of its own success, generating too fast a growth in hash rate that is not sustainable?

Have we already seen peak hash rate?

Disclaimer : apologies if /presumptious/out of place/well considered already/naive/plain stupid/. Just finding out about bitcoin and segwit.

Idea :
- Allow multiple blocks to be added to same parent at same height (block chain gets fatter at current height):
- if blocksize > 0.5 Mbyte;
- if no double spend conflict between blocks.

Node :
- accepts multiple blocks with same parent;
- replaces block hashes with hash of hashes of each block (so block hash no longer "valid" for individual block);
- uses multiple blocks in proof of work.

Miner :
- able to build off of multiple block hash in same way as from a single block hash;
- if block size > 1 Mbyte can filters transactions
  ( i.e only allow inputs within random certain range, determined automatically depending on transaction load;
         with some fix for multiple inputs (1st input))

Rationale : allow blockchain to adjust automatically to arbitrarily large changes in transaction load without changing the block size.