Many, probably most, of the bottom tier tickets do allow you to choose your seats. Most charge extra and that can be $15 or $100 a flight leg. You have to do the math and see which level is most economical for your needs.
Rather than a lot of speculation I googled two trips on the 3rd of March. Boy prices have dropped!
IAH to AMS
United
$366 Basic Economy: Seat Selection for a fee, no upgrades, no refunds, free carryon, checked bag $75
$546 Economy: Free seat selection, Free change, upgrades available, no refunds, free carryon, free checked bag.
$1,444 Economy Plus: Free seat selection, Free change, upgrades available, no refunds, free carryon, free checked bag.
KLM
$712 Light: Seat Selection at checkin, no refunds, changes for $199 fee, free carryon, checked bag for a fee.
$802 Standard: Free Seat Selection, Free changes, no refunds, free carryon, free checked bag.
$934 Standard Plus: Free Seat Selection, Free changes, no refunds, free carryon, free checked bag.
IAH to FRA
Lufthansa
$416 Light: $199 to rebook, no refunds, free carry on, checked bag for a fee, seats when you book
$596 Basic: Free rebooking, no refunds, free carry on, free checked bag, seats when you book.
$2,695 Flex: Free rebooking, refunds, free carry on, free checked bag, seats when you book.
$2,371 Premium Basic: Free rebooking, no refunds, free carry on, TWO free checked bags, seats when you book.
$2,521 Premium Flex: Free rebooking, refunds, free carry on, TWO free checked bags, seats when you book.
$6,000 Business Class