Here's some detailed calculations:
Initial state:
bitcoin: $700
altcoin : $300
total value: $1000
After bitcoin value has dropped about 57%:
bitcoin: $300 (drop about 57%, (700-300)/700)
altcoin: $300
total value: $600 (drop about 40%, (1000-600)/1000)
Needed growth percentage to recover loss without rebalancing:
bitcoin: +133% (400/300)
altcoin: +0%
total value: +67%(400/600)
value after rebalancing:
bitcoin: $420 (600 * 70%)
altcoin: $180 (600 * 30%)
total value: $600
Needed growth percentage to recover loss with rebalancing:
bitcoin: +95%(400/420)
altcoin: +0%
total value: +67%(400/600)
Needed growth percentage difference between with/without rebalancing:
bitcoin: 38% (133%-95%)
altcoin: 0%
total value: 0%
Conclusion:
After rebalancing, it is more easier to recover our total loss by just waiting for a relative lower growth percentage of our lost part.