I have read few posts of persons talking about losing their wallet's containing BTC and couldn't recover it at all. Hence, as people keep losing their BTC due to loss of wallet seed phrase, can it affect or cause a decline in BTC supply entirely?
The lost BTC will have an effect on the number of BTC in supply since it will reduce the amount in circulation when everyone finally gets to know about bitcoin and want to get some. There is 21 million in supply, but that amount will not be available to everyone when bitcoin finally gets accepted and adopted because of how many out of that supply has been lost due to loss of wallet seed. There will still be enough bitcoin to go round, but it will affect the number the available to everyone.