Is it really a must to for investors hold bitcoin for 8 to 10 years, before someone can see profitability in the investment?
Is not a must but a beneficial duration of investment to some persons because your invested money determines how long is going to be, for example you and I have a plan of getting $26k worth of Bitcoin and you started with the weekly investment of $100 and in five years you should be having nothing less than $26k in Bitcoin meanwhile instead of using the same amount you are using I choose $50 based on financial insolvent, so actually in the five years I will be having only $13,044 in Bitcoin while you have meet your target in just five years I'm still half way to it, so to get this I will have to give myself another five years to get there making it 10 years I have spent to achieve it, so actually this example is just a little explanation to show you that 10 year investment is not a must but with your plan it can take up to the years I have explained.
In additional to what you said, another reason why it's advisable to invest and hodli for 8-10 year is to enable you invest profit to keep compounding overtime and that helps to increase your bitcoin investment value. It also give you more time to continue buying and increasing your bitcoin investment compared to when you stop buying early.
Based on your example above, the guy that reached his bitcoin target early can choose to increase his bitcoin target and continue investing in bitcoin because has increased his confidence in bitcoin during his five years accumulation timeline.