Semi-Transparent Oil-Stain - When to Reapply?

Hi All. 
In a nutshell, we have a 30 year old deck that structurely sound.  The wood is not great, but the Benjamin Moore ArborCoat Oil-based Semi-Transparent stain we applied over 4 years ago looked - and still looks - very good.  (I've read about the many nightmares with this stain.) We did apply it to a very dry deck and actually put it on a bit thick.  No peeling at all.  We just pressure-washed it and I was stunned to see there is no peeling and the pigmentation is still very even across all surfaces.  My question...

1) If the pigment that is coating the wood surfaces is still intact and looking good...do I really need to redo the deck at this time?

2) How can I tell if it needs to be stripped off and redone completely?  I hear the "water beading test" is not the true measure of degradation?  What is the proper test with oil-based stains?

3) I hear ArborCoat is hard to remove.  I applied BM's "Remove" stain stripper and found when I scrubbed the wood the stain did not come off...but...when I pressure washed it the stain pigment came of really well.  Any thoughts on this?

4) Considering the deck wood is ancient with a lot of raised grain isn't oil-based stain a better choice?  Seems like anything that would actually penetrate the wood at this point would be best, no?

Thank you very much for any advice that you can provide.

Tom