Nicely put Luc Lion and for the non-believers try to read a bit of reports on glider and small aircraft accidents in Samedan Switzerland.
Nice place to go btw. but tricky with the winds involving narrow turns to downwind in the valley.
So, yes there are reported and causal accidents due to climbing turns into downwind not having enough speed. But then from eyewitness accounts some of the guys turned in 30 deg. bank turns to downwind at some 5 kts above stall speed ...
The one I just found (in German):
www.bfu.admin.ch/common/pdf/1833.pdf
Plus a nice one from my (oh so easy flyable) turboprop times: Approaching LSZH we experienced a nice windshear taking away about 12 kts of IAS. Reported to tower which gave the information to the followin 737. The skipper obviously aware asked about type of aircraft reporting the windshear. Was given "ATR 72". He then reported 17 - 18 kts speed loss at some 2'000 feet AAE.
Shoud answer both:
1st - no not a myth but can kill you if lacking IAS!
2nd - yes inertia plays a vital role.