Signature campaign will give you steady income only till its operating and no signature campaign will survive for life time and when it will disappear you will be left with no income so we cannot rely on signature campaign to get an income so its always good to have multiple sources of income so even if one door closes you will keep on getting income from other methods.
This is very similar to any other job opportunities out there. You're not sure how long the company is going to operate. Of course you can easily get another job, but you're not sure if the competition isn't that much in the field that you're in so it's a lot better to study other fields as well so you can get hired on a different type of company.
Relating to cryptos, we don't know how long signatures can be around. Sure you can just move to another signature campaign when the campaign that you're in closes down. But if the whole signature feature gets removed, then you're going to earn nothing. You should be proactive. You have to anticipate that situation since you never know. Diversify now, learn other stuffs, get a backup plan. Sometimes the backup plan even becomes a more lucrative option.