Right, which is more of a reason to look for TEs. You can't build an offense that uses them as a focal point without at least one viable backup, and if you do and it succeeds, you're either giving them all the leverage in the world or assuming you're going to scrap it as soon as they move on for whatever reason.
Adding one or more backups/heirs apparent/future upgrades is necessary if you're going to build anything around them. Else one of them is just an extremely expensive change of pace/backup, and both have you over an extremely uncomfortable barrel, even though neither is elite.
Two distinct schools of thought in considering team strengths and weaknesses: 1) minimizing exploitable weaknesses vs 2) maximizing strengths. And the second comes in two variants: 2a) maximizing the number of areas of relative strength, and 2b) maximizing the degree of dominance at a given critical area. You could even subdivide 2b) into a single player (say, signing Justin Jefferson to have the advantage in any given 1 on 1) vs making a positional group dominant (say by signing 2 low end #1s, and 3 solid #2s to know that you will always have an advantage in at least one of the 1v1s.)
Obviously it's always a tradeoff among those paths. But it's not smart to design around something you don't have a clear path to locking up or replacing.