Dykes have been build ensuring that a new flood will leave factories dry like (isolated) Islands in the ocean. With the local geology I guess 'inklink' (depression?) has caused the dykes to become less high and a renewed effort (and money) is required.

Up-to-a-point not different from what we do in the Netherlands, but we tend to make those 20-years plans with a bit less ad-hoc activities and try to reserve financing for many years to come. Also an every few years review may lead to minor adjustments and extending the plan beyond the original timeframe. It's a continuous battle and you need government and peoples mindset to ensure that continuity. Current plan has financing reserved till 2025 with reservations for later years being planned

Note that even in the Netherlands after the 1953 flooding it took a few decades to complete a more permanent solution, with the 'permanent' very relative to environment changes, acceptable deaths per 100,000, financial affordability and so.

Water retention was planned, is done with 'monkey cheeks'. When positioned after careful study this "room for the river" can be very effective. Also done in the Netherlands.
www.ruimtevoorderivier.nl/english/

As for fake dykes, no(t much) comment. I understand some ringdykes might still be 95% completed. One may be excused to wonder about the effectiveness. Of course, completed ringdykes which are too low may help to keep the water in, swimming pool like 

Some in another forum accused me and the Thai government of only thinking about Bangkok. In a way a bit like 'London has it all'. To promote industrial activities elsewhere in the country, especially in the areas where farming might always be problematic you need infrastructure. Maybe less than before the Internet, but you still need roads, tracks, aeroports.

Not really a typical Thai problem though.