A few years ago cost structures for managing one's investment portfolios were also significantly higher than today!
There's an even better alternative for someone willing to put in the leg work:
(1) Figure out your investment horizon. For many people, this is way shorter than suggested by generic advice, which makes some diversification beyond "stonks go up" meaningful.
(2) Figure out what costs you'll incur by rebalancing etc.
(3) Write a short script that optimises the amount of activity in portfolio management that improves performance over your investment horizon, given your costs.
Unsurprisingly, the result can vary a lot between people. The result is most likely going to involve a very low level of activity, but the process of finding it out is very informative.
What I've found out (and this is replicated also by more authoritative people like Carver) is that for almost everyone, mixing in some 10--20 % of a safer asset like 10 year bonds and rebalancing yearly outperforms a pure equity portfolio over most realistic investment horizons.