I totally agree with you, 10-20 years is really a long mile of investment and buying consistently and holding this duration is not really easy and if any investor should buy and hold consistently to this years, there is every tendency and possibility that there return or profit is sure and certain because no matter the volatility of Bitcoin it can not correct or continue to be in a low state for this number of years, it will definitely appreciate very well and any investor who has attain this point selling will not be a bad option or idea but if someone must sell, let the money be used for something good because it took them years to achieve that and the reason why they shouldn't sell all is that, it will be very easy for them to start increasing their holding again.
This is a long time but if you have a steady source of income 10-20 year period is normal. If you can gradually increase your average savings then time and Bitcoin holdings will flow in parallel. I think it is relatively easy for those who can keep themselves afloat in the income stream and focus on Bitcoin accumulation. You should consider Bitcoin investment from a long-term perspective and keep depositing continuously. The reality is that you should own a decent amount of assets and have discretionary income from a job/business as well as so your consideration should be accordingly. By being more concerned with the buying point of Bitcoin than the selling point and getting a decent holding until you are able to add Bitcoin to your portfolio from your discretionary income (until retirement).