Are stablecoins worthwhile investing in the long term?

Stablecoins can be quite profitable, so yes, you can regard them as profitable long-term assets to grow your investment. Namely, there are 3 ways you can earn money with stablecoins, including:

• Lending - giving your coins out to a person temporarily, and then earning an interest on them.
• Staking - locking your coins at an online platform like Nexo, Binance, or Coinbase, and then receiving interest.
• Yield farming - staking two pairs of stablecoins (for example: USDT and USDC) and then receiving interest whenever someone swaps one coin for the other.

However, keep in mind that you should only invest in audited & reliable stablecoins, like USDT and USDC. Stablecoins should have a really strong collateral backing them (usually in the currency they're pegged to), as under-collateralized stablecoins can and have crashed before (like the TerraLuna project).