I'm not sure if there was an absence of a plan, but rather an absence of flexibility.
The plan to invest money in the front line, buying the top players from lower league clubs and (presumably) flipping them to reinvest in multiple players wasn't a bad one at all. It is the closest I can think of to an optimal strategy for a team without a genuine sugar daddy.
But there were unforseeables. Many have speculated, and I also think it's the case, that the global financial crisis of 2008 had a big effect. You can trace the expenditures up until that point - SISU were picking up the bill for ongoing losses, paying a hefty wage to a (then) reputable Chris Coleman, paying around £4m for players such as Eastwood, Westwood, Gunnarsson, Fox, Dann, upgrading Ryton, and so forth. But beyond the autumn of 2008, the time of the financial crash, you see a clear case of belt tightening. For example, Bell and McPake were brought in only after Borrowdale and Tabb had been sold.
Then there was the issue of not firing Chris Coleman, though whether that was because RR was too chummy or because they couldn't afford to pay off the contract is anyone's guess. Either way, those sorta things represent the big mistake - a lack of flexibility. You've got to back a plan like that to the hilt, see it through, be prepared to take more losses for a longer-term yield. But if the clients wanted to shut their pocket books then I guess that was their mistake to make.