Originally Posted by Mick
In my opinion, it is easier for a player like NLBwell to improve since he was once an excellent player before the injuries and time off. Bad players who don't have coaches can't fix their forms and usually lack the dedication needed to advance to the next level. It's much tougher for them.
I think the critical part of your statement is "don't have coaches." If someone is coached by a good tennis pro and is very mindful of what they are doing and are willing to work very hard, they certainly can improve significantly in a year or so.
If you don't already know what is correct, without an outside view of what is wrong and, importantly, what to do to improve it, just practicing a lot will only get you so far.
I know a couple of people who were really dedicated, who worked closely with a good pro and moved up to 5.0 in only a couple of years. Yes, it is pretty rare.
Also, a lot of pros aren't that good.