In many respects very true, whilst were seemingly in the mood for StarWars quotations (something I personally dislike)
But all things are true to someone, 'Prom A Certain Point Of View'
The Force itself knows no Light or darkside, The Force simply is.
It us humankind that manifests Light Or Dark (or any shade of grey in-between) thru behaviour and action. We all have a Light Side and a Dark Side the level too which one dominantly leans I guess is what 'labels' us.
So yes, you could warn one about the dangers of leaning in one direction, but is this also not true of leaning too far in the other direction? Is it possible to be too Lightside.
If all things are taken in balance and moderation, then surely it is possible to be too light as well as dark.
Without Light, there is no dark, with no dark there is no light, many things have there equal opposites, tall, short, fat thin, good bad/evil, the list goes on.
Preachy is one term, gently reminders never hurt though
Like in the discussion of emotions, an awareness and acknowledgement of the darkside in all of us is prudent. As you suggested yourself, there comes a point when leaning becomes falling, but is it truly an unrecoverable fall? Im not sure it is, but fear of losing something often prevents the recovery, using Starwars again, vader saw the light (it was blue/green and luke was holding it

) but the Emperor, well, im sure he was pretty much a lost cause, as I suggested, his fear would be the loss of power.
MTFBWY - A