I have a question. Why is SD television worse quality than it was 15 years ago?
The simple answer is it's due to more channels being crammed in to the limited bandwidth available, so the service video bitrates and therefore the picture quality has been reduced over the years to free up space for more channels.
This is particularly the case on Freeview, which has very limited bandwidth, so most channels now run at low bitrates. Excluding the PSB's channels, pretty much all the SD channels now also use the lower 544x576 picture resolution, which lowers the horizontal resolution in an attempt to reduce the compression artifacts caused by the low bitrates. This results in softer SD pictures, especially for 16:9 widescreen programming, which is practically all programming these days.
It's not (as far as I know) a deliberate attempt to convice people to upgrade to HD, it's simply the more channels Arqiva can fit in the better their balance sheet looks.
Satellite has significantly more bandwidth available, and so the SD services on Sky are generally higher quality as they don't have to squeeze the channels in as much.
The fact the majority of us now watch on bigger HD/UHD TV's doesn't help, the upscaling helps to show all these flaws, especially when compared to HD services.