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-calculator

You 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.