Companies give you the sign up bonuses on these cards not because they like you but because they want your custom. For example the American Express Gold card gives you 20,000 (22,000 if you are referred) as a standard option. This is enough for 2-3 return flights in economy within Europe from the UK and within the US using Avios.
Usually you have to hit a spending limit and you get these points. Most of the limits are quite reasonable and possible. What many people hitting the spending limit and then they cancel the card; which means they have the card for a short period of time. I have kept my Amex Gold as my go to card but I have done this with other cards which I know I will not use again as they don’t offer great value. If you feel comfortable to do this you can and when I find good offers I will post them on the website under the deals tab.
Make sure you pay them every month, you do not want to have to pay interest. If this is the case please don’t do this and stick with only one card that is good for you.