Things are become harder than before and arbitrage opportunity is end. Manually process become automated by trading bots which are more faster. Popular altcoins won't allow you to doing arbitrage manually. Cause most of people always keep their eyes on for opportunity and bots are always ready to grab the chance. You can try with undervalued altcoins where you'll see price difference cause they have less volume means less people are trying to make profit from that altcoin. But it'll be risky too cause they can dump a lot
You are right, Arbitrage trading is not more profitable like it used to be that is why people are not doing Arbitrage like before.
Many exchanges are having similar price range because almost all the exchanges are making sure their price in the market should look very close or similar so as to discourage or end Arbitrage trading.
To benefit from Arbitrage trading, you need a good capital, and you will always be ready to take the little profit.