If that's the case then there's no point in having any strategy at all, or doing any research, and just investing in a fully diversified portfolio and sitting on it
I’m not certain if you understand what’s involved in developing a fully diversified portfolio.
The strategy is to develop a portfolio that gives you a level of reward that meets your financial objectives for which you accept for a level of risk that is appropriate to your personal risk profile (i.e. you need to develop a strategy). So you’ve got to calculate your own risk profile; decide on the allocation of your portfolio between equities, bonds, properties, cash, commodities and whatever else to give you a portfolio with an acceptable level of return for the level of risk you are willing to accept; and this involves a degree of research; then you have to find the financial instruments that will make up your allocations; so you need to do some calculation of their performance, standard deviations, etc. and then decide the cheapest way to buy them (another lot of research). Then you do a bit of fine tuning; and then you can ‘sit on it’.
Except every year, as a prudent investor you need, as a matter of course, to review your personal financial situations, and ensure that your portfolio meets your financial objectives. You need to re-balance periodically to maintain your allocations, and if your personal circumstances change (and by implication your risk profile) you need to re-allocate to match your new risk profile, etc. So it’s not ‘no point in having any strategy’ and ‘no point in doing any research’, it’s the opposite.
Alternatively, you could, of course, pay an independent financial adviser to recommend a diversified fund with a good track record, put all your dosh in it, and sit on it.