Defense is only one option and, in SFV, it’s seldom the best option. If you let someone jump in on you, even if you block, they’re just going to keep doing it and keep mixing it up til they get in. The objective is always to eventually do damage, after all.
If you’re about playing footsies, this means giving yourself the best spacing to anti air them and discourage them from jumping. Favour a decent normal, like a high reaching standing light kick, over an uppercut while you’re learning spacing, as it’s usually less punishable if it’s baited out of you and faster to execute.
Learn when it’s “your turn” - what buttons you’ve got that interrupt their string, or allow you to push back at the end of their string (lab time and learning matchups).
Look especially for when they’re being unsafe with their aggression. A lot of low rank aggro players are careless with sweeps and lean towards heavy button specials with longer recovery times, so find a good poke and look for it.
A well placed throw (especially air throws, if you’ve got them) can make someone rethink getting up in your face. Throws tend to have more of a mental effect than a life bar effect (I go for bigger damage, given the choice), so down to preference if you want to roll the dice on throwing as part of your conversion, usually more of an offensive tool, but something to consider.
Be wary that as you shut down parts of a players game, they (usually) start to adapt and change their approach. I’ll tend to let a wildcard Ken player jump around like a maniac and pick moments, rather than shutting down the skies completely, as I don’t normally want to deal with them up close.