OP didn't mention if that bike is still in use or not. If it is no longer used, selling the bike for an investment is a good idea because the longer the bike is left, the more it depreciates in value over time. But if it's his day to day medium, then I would disagree with his opinion. Sustainable investment is from idle money, not from selling other assets to invest.
Investing is not always profitable, and having a job is the surest thing that can guarantee life as well as the future. Do not be too dependent on investment and think that it can bring us wealth without working.
Bikes are not like a real estate or other investment assets which value can appreciate overtime. Another factor which helps their value depreciate more is their material gets deteriorate over time but if you know how to do a maintenance on your bike like if you will paint them to prevent them from rusting then you can prolong their
shelf life. These days in my neighborhoods, people are preferring cycles for commuting nearby as it got health benefits as well. So, selling bikes is somehow a new trend now for various reasons.