1. Blanketing a horse will not stop its coat from growing - it might reduce it a little bit but not enough to matter. If you clip your horse and put the exact right blanket on it for warmth it will still re-grow its coat and need clipping again later on in the season
2. Each horse will have a different threshold for feeling too hot or too cold under its blanket. A horse with a naturally thin winter coat will feel the cold more than one that's got a thick coat, horses with metabolic issues like IRS and Cushings can grow really thick coats but still feel the cold as can older horses so its impossible to say that your horse was toasty under its blanket at a certain temperature so everyone else's should be too.
The one advantage with clipping and blanketing is that it is so easy to regulate how warm/cold your horse is simply by checking it and adding or removing a layer or changing the weight of the blanket. I've often seen unclipped horses that have thick coats sweating on mild winter days and feeling very itchy and uncomfortable