Backups are valid if you have children or poor memory ( like me

)
Jammy fingers do not do DVDs any favours.
Unless you are disciplined, you will ( eventually) loan a friend your DVDs and perhaps forget them.( how many times can you say no??)
Dual layer DVDs are a bit of a hassle( cost and compatibility), and multi dvd players arent exactly flooding the market.
I backup (mainly ) the childrens DVDs and keep the originals as I have no intention of buying them twice.
Unless you are prepared to reauthor the backups to make the main movie fit a DVD5 ( 4.7G) Loss of quality will happen.It varies and may be acceptable
Superbit DVDs back up particularly well even with a little compression
Saying that, Some "normal bit" DVDs ( ex rental Bought off blockbusters) are actually the full uncompressed copy and fit in a DVD5 so these things vary.
I too like crisp PQ and DTS ES/ DD5.1 or DD ex( 6.1): But I only have a 32" HD ready Phillips LCD not a 100" screen

So PQ drop( if any) is bearable ( or subconciously ignored
The DVD player also seems to make a difference in highlighting any drop in PQ from backups due to some compression. Lesser DVD players can make backed up DVDs look unwatchable.
It is doubtful if any DVD changer will feature a
mid range mpeg decoding to.
As you can see there is no perfect solution out there
Whatever you do will have to be a compromise. I would stick to DVD5 with Slight Drop in PQ or: (
take out first dvd replace with second etc...)