It just seems so odd for them to have a season brimming with as much potential as their 18/19 campaign and not want to use it as a platform for more. They've not gone after Fekir again which I thought they would, they've not tried to activate any release clauses or buy any new midfielders, it all seems like a huge risk. The only player I've seen them linked with is Asensio. Their squad is looking quite thin in some areas, especially at full-back. If TAA or Robertson get injured they're pretty screwed as of right now.
This is the biggest thing to me. They can look back on a huge point tally and a CL win and be happy with that, but there seems to be no self awareness to the fact that other than Oxlade-Chamberlain and Gomez they didn't really suffer from many injuries. If they go into next year with relatively no new faces then they are banking on having the same luck with injuries. For the front three, and as you said fullbacks, they'd have no shot at the title if they sustained multiple injuries.
The kitty might be empty. Also, they are comfortably the 2nd best team in the league so a Top2 finish is all but guaranteed so what would be the point of doing another £150mil with no guarantee of improvement?
If they do end up not signing anyone and Spurs bring in a couple more players (and hold onto Eriksen and/or Alderweireld), I could absolutely see Spurs finishing second barring they stay relatively fit. I don't see Spurs having as deep of a CL run next year and on paper they should be a better, more consistent side in the league.