It really depends on the type of fish you are trying to keep. Truth is, not all fish live the same amount of time. Take Killie fish for instance, most, if not all, are annual fish meaning that they only live for about a year (from egg). Other fish like your larger SA Cichlids, I've kept for 8 to 10 years. Also, keep in mind that with all the hybridizing and inbreeding that has happened over the years, today's tank raised/ created fish may not be as strong as their wild counterparts so they are not lasting as long.
What I discovered long ago was that the key to longevity of the fish was stability. (My favorite expression is " The less you mess, the better the success.") I did not overcrowd my tanks so weekly water changes of more than 10% -15% of volume were rare for me but my water remained well within the ranges necessary for my fish to frequently breed. A good balanced diet was also key. Supplement flake and dried foods with live weekly.
As for your getting bored with your selection, if you think that is going to happen then you are choosing the wrong fish to keep. (IMO
) If you are picking a fish that has a known lifespan of say 5 years, you should be doing what you can to have that fish for 5 years. If you don't want it that long, maybe pick something else that has a shorter lifespan so that you can change your selection frequently. (again IMO)
My suggestion would be to research the fish you are intending on buying (before you buy) and see if they are a hardy specie or a long lived variety or if they are recommended more for advanced hobbyists or beginners, etc. Also try to make sure you set up an environment for these fish that are to the fish's liking. (Meaning: don't put a fish that likes to hide in a bare tank with no decorations or a fish that needs a lot of swimming space into a tank that is overcrowded with decorations.)
I hope this helps some...