From a technical point of view, the area around seams under tension goes transparent first because the lycra fibres which provide the opaquesness to the fabric are most stretched in this region and therefore most vulnerable - it's not only chlorine and sun that damages lycra, components of sweat will too.
Also, on less well made shorts (this probably doesn't apply to Assos) the cut edge of the lycra is too narrow so the lycra fibres (which are woven into the flat nylon fabric while it's being woven) escape from the seam - you can often see a line of white lycra fibre ends along the seam line after the first wearing.
There is definitely a trend to thinner fabrics but even heavyweight lycra will become quite see-thru once the lycra (which is only latex spun into a filament) decays. There are other stretch fibres but sadly none which combine good skin feel, low cost, convenience of manufacture and reasonable longevity.
Finding a make/pair of shorts cut somewhat more generously around the bum to avoid overstretch might be one long-term solution. Careful washing and drying out of the sun/heat will prolong the life of existing lycra.
Or buy longer jerseys