my 2 cents again

It all boils down to cost, investment vs return.

It is very rare to find a tiny little hole in the copper pipes.  The usual suspects are service valves, pipe joints, condensor coil.  The most likely leak point are the low side/high side service valves.  The seal on the valve core is made of rubber/plastics.  The material can deteriorate over time.  Replacing valve core is very simple.  The valve core costs very little money.  However, you will have a hard time to find any contractor willing to change the valve core for you.  It's very small money <$100.  They would rather change the whole system for you.  To locate the leak, you will need a sniffer.  Most AC contractors don't even carry a sniffer today.  Again, they don't care about small jobs .

If installing a new system, you have to ask the contractor to do a pressure test first.  At minimum make sure the line sets can hold vacuum, meaning there is no leak in the line sets.  Purge the line sets with liquid nitrigon.  Also, watch how they welded the pipes of the new system.  If not careful, the service valves of the new outdoor unit can be damaged by the extreme heat during welding.  In fact, a "wounded" service valve is the most common cause of small leaks.  And it's almost not practical to replace a whole service valve (not the valve core), due to the high cost and lack of room to cut/weld new piping.  It requires much higher skills that most AC contractors don't have.