Entering through France is definitely not a requirement. Even if you had planned to enter through the Netherlands from the get go, if you are staying in France the longest, getting a French visa is what you were supposed to do.
Changing your plans compared to the itinerary you submitted with your application is not completely forbidden either but it does make your situation a little bit more difficult. Depending on the details (including whether you have used this visa or stayed in the Schengen area before), it could in theory raise questions.
Ultimately, border guards have the power to deny entry or even cancel a visa. They would not do that merely because you enter through the “wrong” country but only if they think you got a French visa to game the system or want to do something illegal like working in the Netherlands. That's probably the source of the stories you heard.
Because of this, buying a ticket at the airport would not help as it's not a matter of fulfilling some formal bureaucratic requirement. The question is whether border guards believe you intend to go to France, do what you claimed you would do and generally respect the rules of the Schengen area. If they do believe you, it's OK, you don't need a ticket. But if they don't, it's too late, they won't let themselves be swayed by your willingness to spend some money to buy a ticket on the spot (who wouldn't when faced with the prospect of immediate removal?).
On the other hand, buying a plane or train ticket to France in advance, while not required, could be a good idea and add to your credibility if challenged. Having other documents showing you will really go to France at hand would also be useful. Same thing for a return ticket leaving from France (if that's what you want to do) and, of course, a good explanation for your change of plans.
But only do that if you really plan to use that ticket and do not lie at the border; You risk getting trapped in your own lies and giving the appearance that you are trying to cheat, which would greatly increase the likelihood of a negative outcome.
See also the following questions for a full review of the rules and some issues people have faced in practice: