Darryl’s Analysis

Environment:
This is a good sized window – it looks fine for a Dracaena marginata.

Effort:
Your watering strategy – the WHEN to water and the HOW to water – all seem fine but I see two issues you should address:

1. Your soil appears to be of poor porosity, which may prevent oxygen from reaching the lower parts of the root system.  This should be addressed the next time you repot the plant by adding some perlite or bark chips – approximately 1 part perlite/bark chips to 3 parts potting soil should be fine.
2. You said you never used fertilizer: if you don’t, the number of leaves on the plant will eventually be “very few” after another year or so.  Older leaves will die off at some point but they’ll die off sooner and quicker without fertilizer.  Here’s what I use: 3-1-2 fertilizers.

Expectations:
Unfortunately, once yellowing begins on lower leaves (meaning, the oldest leaves), there’s no way to reverse it.  But even if you correct the two things mentioned above (soil porosity and fertilizer use), that doesn’t mean you’ll never lose leaves again – it just means they will fall off at a slower rate while new leaves grow.

The most critical thing you should embrace is this: leaves have a limited lifespan.  That means they will die at some point regardless of how “good” your conditions/care are.  The goal is to ensure you have the best possible conditions and care efforts so the plant keeps putting up NEW growth.

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