Outgrowing your equipment is a very real occurrence that happens to just about all of us! If I could go back about 25 years and start over, there are some purchases that I never would have made. Especially when it comes to my boil pots. I just upgraded from a 5 gallon system to a system capable of making 5-10-15 and even 20 gallon batches. The 13 gallon brewpot that I figured was large enough, isn't even capable of 10 gallon batches. I'm now purchasing a 15 gallon keg that is already drilled and coverted to a boil pot. I'm getting it for $60, which is a killer deal.
I will rarely, if ever make 20 gallon batches. When I make 15 gallon batches, they'll most likely be parti-gyles and will require two separate boils anyhow.
If I do brew a 20 gallon batch, it will be a back to back boils. Although, I do have my original 8 gallon brewpot too. My 20 gallon batch could be broken up into boils of 10 gallons, 5 gallons and 5 gallons. All I need now is two more burners. I only have the one burner for now, so I'll eventually need at least one more.
As you can see from my rambling, no matter how long you've been doing this, it always seems that you're planning ahead and still not sure, even then, if you're making the correct purchases.
The best part is that there will always be people looking for equipment. So, when you outgrow something, you can usually find someone to sell it to or trade it to, to offset the cost a little bit.