It doesn't matter whether you take:
$5K x equity growth + $5K x bond growth, or
($10K x equity growth + $10K x bond growth) x 1/2
Same result.
Multiplication is distributive over addition.
You seem to be missing the overwhelming significance of not rebalancing. To simplify the arithmetic and make the effect easy to see over a few iterations (
years), let's use these exaggerated hypothetical returns:
Each year the equity fund doubles. Each year the bond fund returns 10%. Each year the hybrid fund returns 60%. I think you'll agree that aside from the magnitudes, I've preserved the general relationship - largest returns for equity, hybrid fund returning just over the average of the equity and bond returns.
We'll use $5K for each of the equity and bond funds, since then we don't have to divide by 2 at the end of every step. Keeps arithmetic simpler.
Year 0: $5K equity + $5K bond, vs. $10K hybrid
Year 1: $10K equity + $5.5K bond, vs. $16K hybrid. Hybrid ahead by $0.5K.
Rebalance - we're trying to emulate a hybrid fund, so we rebalance for the same allocation
Year 1: $7.75K equity + $7.75K bond, vs $16K hybrid.
Year 2: $15.5K equity + $8.525K bond, vs. $25.6K hybrid. Hybrid ahead by $0.975K
Rebalance.
Year 2: $12.0125K equity + $12.0125K bond, vs. $25.6K hybrid.
Year 3: $24.025K equity + $14.21375K bond, vs. $40.96K
At this point, the hybrid is ahead by $3.72125K.
The hybrid lead is accelerating, as one would expect because its yearly outperformance of 5% (60% vs. the average 55% of the stock and bond funds) compounds year after year.
You can't say on the one hand that you want to keep a constant 50/50 mix (or 40/60 or whatever), and on the other hand say that you don't care, you're going to let your portfolio become equity heavy. Look at the actual fund figures. Without rebalancing, the portfolio evolves into one that's over 90% equity ($2,478,356 vs. $215,945).
In the 46th year, this 90/10 portfolio will pull away even further from the 40/60 hybrid. But 90/10 is not the mix that the investor wants to hold, year in, year out.