Bitcoin mining is not profitable forever because the difficulty keeps on increasing over time. The difficulty ensures only one block is found every 10 minutes; the difficulty gets adjusted every 2016 blocks, or every 2 weeks.
You can try this mining profitability calculator which includes the current difficulty:
https://www.coinwarz.com/calculators/bitcoin-mining-calculatorYou can view a historical difficulty chart here (the difficulty is the red line):
https://bitcoinwisdom.com/bitcoin/difficulty - that page also lists the prediction for the next difficulty (at the top "Estimated Next Difficulty").
The amount of computing resources (hashrate) you need increases as the difficulty increases, so you need to invest in new hardware with a higher hashrate.
I have to disagree. Mining will always be profitable for some people.
Mining is getting less profitable. I, for one, consider BTC mining to be unprofitable. At $0.10 a kW/hr it doesn't make sense for me to buy an S11. I will never make back my initial investment.
But there will come a time that BTC mining will become less and less profitable even for the big guys. At that time people will stop investing in SHA256 ASICs and put their investment dollars elsewhere. As that happens the diff rate will begin to drop. At some point there will be an equilibrium when the diff rate will be roughly constant.
Ultimately if it's not profitable to mine; and no one mines; then the coin fails.