shieldgrab is not a very viable option if the player knows how to space aerials
Shield pressure tends to be impromptu since if they actually expect a shield they have better options do deal with it than hitting it, or hitting just outside of its range (unless they're trying to cover for spotdodges as well). If you were sliding towards them in shield, or ran at them and suddenly shielded, it would be significantly harder to safely space an aerial than if the opponent just stood there in shield and let them take their time. If they aren't assuming or preparing for shield (which if they do, opens them to being whiff punished on descent or landing if the opponent doesn't), they won't necessarily try to space attacks for shield pressure purposes. Also, shield DI exists, and can eclipse the distance one can travel horizontally on descent.
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Ideally, don't get into the situations where you have to shield in the first place, or insert your shielding where their attack is most frame negative. Sheik spacing fairs at your shield? Instead of shielding, take a page from what some other characters do, crouch or move back a little, and downtilt Sheik's landing (if it was spaced to barely hit a shield, it will definitely miss Falco, for instance, moreso if it was meant to beat an input of shield DI), or ASDI down and in and grab or Shine. Altenatively, back air as she rises, roll or wavedash under her, or usmash or utilt (facing away) before the fair could come out. If you feel you have to shield, shield DI out to avoid mixups after something hits, if it's relatively frame safe, and shield DI in to be able to retaliate better. Same principle applies to Fox. Likewise, back air, utilt, and upsmash can beat him jumping in (as can dair), but these are fairly likely to trade. However, if he's in the air, he can't CC/ASDI down while you can for any of these but dair or bair, so you can, so you can get a combo off of it. You can also CC shine, or CC double shine in anticipation of them holding down to shine on landing.
If you don't feel very safe without shielding, just wavedash out of shield quickly if nothing's happening. Generally not into the opponent unless you have ample room or really think you can catch them with a shine (though if you make them respect this as an option, covering your retreat while being ready for it is astronomically difficult). If the opponent jumps at you, is decently close, and you react quickly, you can roll through them. If you were in the corner, this means stage position. Alternatively, you can work of a clean ledge game and space invincible ledgedashes to either put you in range to hit if you know you can go the distance, or just out of their range to punish an attempt to outspace a ledgedash approach.
Also consider what the opponent does after an aerial, pressuring that way. If they jab, just mashing shield grab while holding down and in (if done correctly) will beat this until the jab starts to knock you down (which is like, past 100%).
Consider the opponent's percent when acting OoS. From grab or shine, you can at the very least, escape any opportunity for the opponent to hit you for landing one of these, but realistically can get decent damage. If the opponent is grounded when the attack connects, aerials OoS and usmash could potentially get you hit if the opponent holds down. Dair, if done before the opponent had the opportunity to shield, will almost certainly result in the opponent being able to act before your landing lag ends, if not before you even land. At lower percents, given how limited options seem to look if the opponent CCs, it may be better not to retaliate, but to just get out of the way, or again, avoid having to shield in the first place.
Also, practice retreating laser downtilt/turn around utilt. If the opponent was spacing for a shield or CC or something, catching them with a laser before they land into either tilt can start a combo to build percent and open more options to you. This can be especially useful vs Sheik, who has relatively slow jumps compared to Fox, and while she can jump over the lasers, if she lands on them, after they've gone further out, it means you could potentially combo something that hits much harder off the laser. I'm not saying to do this one out of shield however, but in anticipation of, or upon seeing a jump spaced for a certain range (and ideally not to close to you).
Finally, if you're sure they're going for an attack meant to end up on top of you, you can take it back to 2001 and just time a spotdodge towards the point they land, or the start up of a grounded move. If you don't have confidence in your timing, or that you'll be able to hit them afterward, you're better off rolling. Spotdodge eliminates the battle between shield stun and endlag of attacks, so only the lag is relevant. Under ideal conditions, if Fox nairs and lands just as invincibility ends, and doesn't miss his L-cancel, you can act on the same frame. Any other aerial, you act first. Realistically, you have a few advantages going into this. If Fox was nairing, there's a decent chance he wasn't expecting a spotdodge, and a high chance he wasn't expecting to miss. Moreover, if hitting a shield was at all expected, you would be pushed back to some degree, unless you overcame this completely with shield DI. You're altering the any change in range he's expecting, but also, making him miss an attack at that range. If he was doing an extemely low nair, this probably doesn't change much, but for higher heights, you can mess up both the fast fall and L-cancel timing. Of course, this isn't a technique I recommend for going against Fox's nair in particular, active frames counter spotdodges. This has the potential to be much more useful vs aerials with lower active frames, higher lag, or higher shield stun, however. But the most important detail is that you don't have to act out of shield if this works. You can do any option.