I know many will say the time ended, we must have bought before 15 days but I think this is the best time. Before couple of weeks ago, BTC was around $4k and which is why altcoins are overpriced in term of BTC. Now that BTC has gained price, altcoins price has decreased in term of BTC and now is the best time to pick your favorite altcoin because right after the BTC ride, altcoin will also follow the trend.
Yes, you're quite right because many altcoins were underlying on bitcoin, thats why many crypto lovers will happy when bitcoin increasing again. And I think these days since the recently passed were the best to buy top altcoins because it will drive into huge profits, once get up high again , I mean bullrun.