This would be possible if the person is earning way higher than $60k per year or luckily Bitcoin price drops down to $50k or they would have to increase the yearly intervals they have set to accumulate the 4.5 Bitcoin.
I am not going to presume that the bitcoin price is going to drop... sure it might, but we should not be making our plans based on things that are out of our control.. Yet, each of us should be able to figure out how much of our income that we can put into bitcoin and then therefore how much bitcoin we might speculate that we are able to accumulate through our various accumulation strategies.
If we are new to something, we also might need to be more measured in terms of working our way up to a certain budget, and so hopefully we are not making large mistakes in our ways of calculating how much bitcoin that we might be able to buy or even that we are not making mistakes in regards to other aspects of our cashflow management and various back up funds that we also might need to build and/or maintain.
You are right, sir, I believe that it's more better to invest flexibly than getting oneself in a bad depressive state for the sake of meeting up. Like you mentioned, there are some people that can invest aggressively and still the able to catch up with their pattern of successfully handling the investment and keep it consistent but some persons can not.
Price drop should not be a limiting problem for why the person should not invest but even if the price drops, it's just an advantage to accumulate more than what was expected.