Typically, galvanized steel DWV lines at age 50 years will be like new in the vertical portions and badly deteriorated in the horizontal sections. One approach is first to check the horizontal sections for proper pitch. Due to errors at initial installation, the pipes may have inadequate pitch or the house may have settled affecting the pitch. Inadequate pitch in galvanized drains hastens deterioration. To correct horizontal pitch typically involves changes in the vertical sections and is a reason to redo the whole system. Where existing pitch is adequate, replacing just the horizontal sections with PVC is one option that typically works well.

I routinely replace any 30+ years old galvanized steel line if I have a wall already open.

Cast iron stacks are usually fine, even when they hit 100 years, though I will replace smaller cast iron lines if the wall is open.

Pitch seems fine and the horizontal runs are short--1.5' to the drop to the basement and another 4' (accessible from the basement) to the 1.5" line that carries to the sewer. Horizontal runs are all solid after tapping with a hammer, so I guess my descretion is to leave it behind and seal the wall--doesn't seem worth messing with. Thanks for the feedback--If anyone has a differing perspective from the ones provide, please chime in.

The only place worth tapping is in the middle of the bottem of the pipe, whack it a good one and if it doesn;t break its ok for strenght but still probably plugging up on the inside.

I spent $20 on 30+ foot of 1 1/2" pipes and fittings that the previous owner spent $100 on a plumber to snake. The cost is minimal right now to replace.

I had a 1952 update about a year ago.
When the kitchen drain clogged it was cleared.
The clearing eliminated the build up in the pipe and a 12 inch horizontal piece proceeded to leak all over.
I keep it around to show anyone who trusts older galvanized steel pipe.

It had a series of holes along the entire length.

For me this is a no brainer. At 50 years old that galvanized has long since past its life expectancy and given that you have the walls opened up, there is no time like the present to ward off a future problem that would be very expensive to correct.

Unless there is a local code to the contrary you can remove the galvanized and replace it with PVC.

I am also wondering why they have a 2" vent on an 1-1/2" line? Under the IRC a vent line may be reduced to 1/2 the diameter of the line it serves but not less than 1-1/4". Under the UPC vent lines are sized by the total number of DFU's on the drain line, but given that this is a 1-1/2" line the DFU load should not exceed 3dfu's and an 1-1/2" vent is rated for up to 8dfu's.

The only reason I can think of for increasing the size of the vent line is if the total developed length of the vent exceeds 40' we are required to increase it by one nominal trade size, which would be the 2" you have.